Break broccoli and cauliflower into small florets and remove thick pieces of the stalks.
Blanch florets in boiling water for 3-4 minutes.
Remove them from heat and set aside.
Preheat oven to 375.
Melt 4 tablespoons of butter in a large skillet over medium heat.
Cook onion and garlic 3-4 minutes until softened.
Season with salt, pepper and crushed red pepper.
Sprinkle flour over onion mixture and whisk to make sure flour is absorbed.
Cook for another minute.
Gradually add in broth, whisking constantly.
Cook until the sauce thickens.
Gradually add in milk, whisking constantly.
Combine the breadcrumbs and 1 tablespoon melted butter, and stir until blended.
Layer 1/2 of the broccoli and cauliflower in a small casserole dish.
Pour 1/2 of the sauce an top of the vegetables.
Sprinkle 1/2 of the cheese on top of the sauce.
Layer on remaining broccoli and cauliflower.
Pour remaining sauce an top of the vegetables.
Sprinkle remaining cheese on top of the sauce.
Sprinkle breadcrumbs evenly over top of the casserole.
Bake for 25 minutes or until cheese is melted and bubbling around the edges.