Season cauliflower and broccoli with salt and pepper. Roast in a 400°F oven for 8-10 minutes -OR- steam in a large skillet with a lid and 1/3 cup water over medium heat for 5-6 minutes until crisp-tender. (Add more water, if needed)
While the vegetables are cooking, melt 2 1/2 tablespoons butter In a medium saucepan. Use this melted butter to brush in the baking dish and then transfer vegetables to the baking pan in a single layer. Heat the broiler and position rack 8-10 inches from the heating element. Note: If you don't have a broiler-safe dish, bake on convection bake at 400°F - this will brown the top more efficiently without overcooking the cheese sauce.
Return the remaining butter to the stove and heat until bubbly. Add 2 tablespoons flour and whisk over medium heat until bubbly. Continue to cook on medium-low for 3-4 minutes to incorporate the flour fully.
Reduce heat to low and whisk in 1 cup of milk, 1/2 teaspoon Dijon, and a pinch of cayenne; continue to stir and gently simmer on low about 5 minutes or until sauce thickens. Remove from heat and stir-in 1 cup cubed Brie and 3/4 cup cheddar cheese, continuing to stir until cheese is melted. Taste and season with salt and pepper.
Pour the sauce over the vegetables and sprinkle with Parmesan and bread crumbs. Spray crumbs lightly with cooking spray and broil for 10 minutes (depending on broiler - watch closely) until the cheese is bubbly and topping is lightly browned in spots.
Make Ahead TIP: Make ahead and cover for up to an hour. Dress with the bread crumbs and Parmesan just before putting in the oven.
Cheese TIP: Brie has a soft white rind that is fine to add to the sauce, however, remove any dry edges as they do not melt as well. The soft rind may not completely melt, but that's okay and not noticeable in the final dish.