Preheat the oven to 375°. Spray a small casserole dish with vegetable spray and set aside.
Trim the end from the cauliflower and cut it vertically into 1" slabs. Trim the florets from the stem and then cut the stem into bite sized pieces. Add about ½" water to a large pot and fit it with a steamer basket. Bring the water to a boil and place the cauliflower into the basket. Steam for about 10 minutes or until tender. Set aside.
In a medium saucepan, heat the butter over medium high heat. When butter is melted and bubbly, whisk in the flour, nutmeg, dry mustard, cayenne pepper, kosher salt and black pepper. Stir until the butter and dry ingredients form a paste or roux. Slowly add the milk, whisking constantly. Cook, stirring and scraping the sides and edges until the milk mixture thickens and comes to a low boil. Cook for one minute and remove from heat.
Add ¾ cup cheddar cheese and ½ cup gouda, stir until cheese is melted. Stir in the whole grain dijon and set aside.
Spoon ¼ cup cheese sauce into the bottom of the casserole and spread evenly. Use a slotted spoon to transfer the cauliflower to the casserole and arrange in an even layer. Top with cheese sauce. Sprinkle on the remaining cheddar cheese and bread crumbs.
Bake for 18-20 minutes until bubbly, then turn on the broiler to high and cook for 1-2 more minutes or until breadcrumbs are nicely browned. Remove from oven and rest for 5-10 minutes before serving.
Notes
Reheat leftovers in an oven at 350° for 10-15 minutes.