Combine parsnips, carrots, potatoes and onion on a large sheet pan. Add rosemary, thyme, crushed red pepper (if using), salt and pepper.
Drizzle with olive oil. Using your clean hands, toss vegetables and herbs with olive oil, until well coated with oil, herbs and spice. Spread into a single layer and bake for 20 minutes.
Using a spatula flip vegetables and place back in the oven for an additional 10-15 minutes until golden and tender.
Note: if the vegetables are scrunched tight on the pan, and your intent is to get some browning and caramelization, you may want to transfer about ⅓ of the veg to another pan to finish cooking. Spreading out the vegetables allows for better browning.