Adapted from Eating Well.

Ingredients
8 cups prepped vegtables, choosing one line from the following:

• 1 broccoli crown and 1/2 head cauliflower, cut into 1-inch florets
• 1 large butternut squash, peeled and cut into 1/4 inch slices
• 2 1/2 pounds brussels sprouts, trimmed off stalk and halved
• 5 large carrots and 5 large parsnips, peeled and wooden core removed from parsnips, cut into 1/4-in slices
• 2 1/2 lbs green beans, trimmed and cut into 1- to 2-inch pieces

1 medium onion, thinly sliced
2 Tbsp white all-purpose flour
3/4 tsp salt (optional)
1/4 tsp white or black pepper
2 1/2 cups plant milk
1 1/2 cups fresh whole-wheat breadcrumbs
Optional: 1/3 cup nutritional yeast and/or 1 Tbsp chopped fresh herbs

Preparation
Position racks in upper and lower third of oven and preheat to 425F.
Divide vegetables between two baking sheets and spread in an even layer. Roast the veggies, stirring once and rotating the pans top to bottom half-way through, until tender and beginning to brown, 20 to 25 minutes.
Meanwhile, water sauté onion until very soft, 5-8 minutes. Add flour, salt, and pepper; cook, stirring, for 1 minute more. Add milk and continue to stir, scraping up any brown bits. Cook, stirring, until the sauce bubbles and thickens enough to coat the back of a spoon, about 4 minutes. Remove from the heat. If adding nutritional yeast or herbs, stir them into the sauce.
When the vegetables are done, remove from oven. Preheat the broiler.
Transfer half the vegetables to a 2-quart, broiler-safe baking dish. Spread half the sauce over the vegetables. Add the remaining vegetables and top with the remaining sauce.
Sprinkle the breadcrumbs over the gratin. Place under the broiler and broil, watching closely, until the gratin is bubbling and beginning to brown on top, 1-5 minutes. Let stand 10 minutes before serving.