Preheat Oven: Preheat your oven to 375°F (190°C). Lightly grease a baking dish.
Prepare Vegetables: Steam or blanch the broccoli and cauliflower florets for about 4-5 minutes until slightly tender but still crisp. Drain well and set aside.
Make Cheese Sauce: In a medium saucepan, melt the butter over medium heat. Stir in the flour and cook for 1-2 minutes, stirring constantly, until it forms a paste (this is a roux). Slowly whisk in the milk, stirring until the sauce thickens, about 3-4 minutes.
Add Cheese: Once the sauce has thickened, remove from heat and stir in 1 cup of cheddar cheese, Parmesan cheese, garlic powder, onion powder, and season with salt and pepper. Mix until the cheese is fully melted and smooth.
Assemble the Bake: Place the broccoli and cauliflower florets into the prepared baking dish. Pour the cheese sauce evenly over the vegetables, gently tossing to coat everything.
Top with Cheese and Breadcrumbs: Sprinkle the remaining ½ cup of cheddar cheese over the top. If using breadcrumbs, sprinkle them on as well for a crispy topping.
Bake: Place the dish in the preheated oven and bake for 20-25 minutes, or until the top is golden and bubbly.
Serve: Remove from the oven, let cool slightly, and garnish with fresh parsley if desired. Serve warm and enjoy!