Rutabaga & Butternut Squash Casserole

Looking for a cheesy vegetable gratin to go with everything from rotisserie chicken to Easter ham? This tender rutabaga and butternut squash casserole is one of the best. I use thinly sliced root vegetables with leeks and fennel in this baked butternut squash recipe. The roasted rutabaga and butternut slices get tender and sweet nestled in a mix of creamy white sauce and cheese.

This post has been updated for recipe, photos and content since its original publication in 2017.

Butternut squash is so versatile.

I always seem to have a butternut squash sitting on my kitchen counter because it goes well with so many things and my family loves it.

In this butternut squash casserole, it adds a light honeyed note that plays well with the vegetal quality of roasted rutabaga.

Honestly though, butternut squash is a mainstay for me because of its versatility.

I use this bulbous veg in everything from my Thai squash soup to this sausage and pumpkin favorite. Butternut squash puree makes my macaroni and cheese a little healthier, sweeter and gives it that neon-orange “Velveeta-like” hue.

It’s great as a simple roasted side dish, in a luscious risotto or healthy farro pilaf. And don’t get me started on this heavenly butternut squash lasagna. OMG!

It’s also cheap, plentiful and good for you! Can’t beat that!

Ingredients you’ll need for rutabaga and butternut squash casserole

Components for the butternut squash casserole

This is a simple baked butternut squash recipe, but there are a few components that need to be prepped before assembling the vegetable gratin. starting with the leeks and fennel.

Leeks

Leeks are a root vegetable related to onions and other alliums but with a softer milder flavor, especially when cooked slowly over a low heat. Simmered gently with a little oil and/or butter and a pinch of salt, will release leeks natural moisture and bring out their sweetness.

It’s important to clean leeks well before using them as there’s often dirt and detritus that grows between their layers.

To clean leeks:

1. Remove the root end and cut away the dark, woody leaves, leaving only the white and pale green parts.
2. Slice the leeks crosswise into 1/4″-1/2″ slices and transfer to a bowl of cold water.
3. Separate the rings of leek and swish them with your hand to dislodge any dirt. (Dirt will sink to the bottom).
4. Lift the leeks out of the water and transfer to a salad spinner to dry.

Fennel

Fennel is a perennial herb with spindly fronds and a bulbous stem. The bulb and fern-like fronds are edible, but the stems tend to be tough, so I don’t recommend using them. Fennel has a light licorice or anise flavor that’s more pronounced when served raw and softens when cooked.

To trim leeks:

1. Cut away the stems and fronds.
2. Slice the bulb into quarters, vertically and cut away the tough inner core.
3. Lay the fennel flat on a cutting board and cut 1/4″ slices.

Melting Leeks and Fennel for the baked Butternut squash recipe

Ok, so you’re not actually “melting” these veggies. However, cooking them with a little fat and salt, in a covered skillet over a low heat, will soften the root vegetables to the point where they slump and don’t fight back. Here’s how to do it.

1. Add butter and olive oil to a wide skillet with a tight fitting lid and heat over medium to medium-low heat.
2. Add the sliced leeks and fennel and sprinkle with kosher salt, pepper and thyme.
3. Stir to combine, cover and simmer for 10-12 minutes, stirring occasionally until the root vegetables are very soft and tender.

You don’t want to cook on a high heat, as that will brown the veggies. Be patient and go low and slow.

A béchamel is just a fancy name for a creamy white sauce. It’s what I use for my scalloped potatoes and as the base for my cheese sauce in my macaroni and cheese. Béchamel sauce is thick and creamy and really simple to make.

How to make béchamel for the vegetable gratin

1. Melt the butter in a saucepan over medium to medium high heat.
2. Sprinkle in the flour, salt, white pepper (if using), mustard powder and garlic powder and stir together until it forms a bubbly paste and there’s no dry bits of flour. Cook for about one minute, stirring constantly. (This mixture is called a roux, and it will help to thicken the sauce).
3. Add warm milk a little at a time to the flour mixture, stirring constantly. (It will be thick and pasty at first, but just continue to stir and add milk. The roux will melt into the milk.
4. Continue to stir constantly as you bring the mixture to a rolling boil and cook for one minute, then remove from the heat and set aside.

The sauce is lush and creamy. As it sits, it may develop a “skin”, but just give it a quick stir to blend it back into the béchamel.

How to prep root vegetables for the casserole

1. Peel the butternut squash and rutabaga and trim away the ends.
2. Slice the butternut squash in half vertically, and scoop out the seeds. Discard.
3. Slice the rutabaga and butternut squash into very thin slices. (Note: rutabagas tend to be a little harder than the squash, so I recommend cutting the rutabaga in half and laying them flat side down, to give you a solid base to slice.

You want the butternut squash and rutabaga to be roughly the same thickness so that they bake in the oven at about the same time.

Layer the vegetable gratin in a 2-quart casserole dish.

Assembling the rutabaga butternut squash casserole

1. Spread about 1/3 cup of the white sauce on the bottom of the casserole dish, then layer the rutabaga and butternut squash slices (alternating for color) to cover the bottom of the dish.
2. Top evenly with 1/3 of the melted leeks and fennel.
3. Sprinkle on 1/3 of the cheese and 1/3 of the remaining bechamel sauce.
4. Repeat the layers 2 more times finishing with bechamel and cheese.
5. Cut a piece of foil large enough to cover the gratin dish and spray it with vegetable spray (to prevent the cheese and sauce from sticking to the foil when you’re baking the butternut squash casserole.
6. Bake at 375° F for one hour.
7. Reduce the heat to 350° F and remove the foil.
8. Continue to bake the vegetable gratin for 25-30 more minutes or until bubbly and lightly browned.

Why is my vegetable gratin watery?

Though butternut squash and rutabaga don’t seem to have much liquid when you’re prepping them, there’s a surprising amount of moisture that will seep out during baking.

Pro-Tip: After the gratin sits for a few minutes, you’ll notice watery puddles around the edges. It’s fine, but if you want to remove it, scoop it off with a spoon or use a turkey baster to suck up excess moisture.

Swaps & Substitutions

Though the rutabaga and butternut squash are delicious in this vegetable gratin, you can also substitute other root vegetables.

• Swap Yukon Gold potatoes for the rutabaga or butternut squash.
• Use sweet potatoes instead of the squash.
• Try sliced turnips instead of rutabaga.
• Use other types of winter squash like acorn, delicata or baby pumpkin.

FAQ’s

What to serve with root vegetable gratin

• Cider Brined Smoked Pork Loin
• Grilled Herb Crusted Rack of Lamb
• Marinated Roast Turkey Breast
• Beer Can Chicken
• Grilled Veal Chops
• Rotisserie Chicken At Home
• Bourbon Marinated Grilled Flank Steak
• Herb Roasted Chicken with Pan Gravy
• Pan Roasted Chicken Breasts with Mustard Sauce
• Homemade Roast Beef with Onion Gravy

More vegetable gratins you’ll love:

• Kale Barley Mushroom Gratin
• Cauliflower Mustard Gratin
• Creamy Chayote Gratin
• Savory Sweet Potato Casserole
• Cauliflower and Mushroom Gratin
• Kale and Butternut Squash Gratin
• Twice Baked Sweet Potato Gratin
• Cheesy Tomato and Egg Gratin
• Potato Ham Au Gratin

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