Steam or blanch the frozen broccoli and cauliflower florets. Drain well and set aside.
In a saucepan, melt butter over medium heat. Add flour and whisk continuously for 1-2 minutes to create a roux. Gradually add milk to the roux, whisking constantly to avoid lumps. Cook until the mixture thickens.
Reduce heat to low, add shredded cheddar cheese, grated Parmesan cheese, garlic powder, salt, and pepper. Stir until the cheeses are melted and the sauce is smooth.
In a large mixing bowl, combine the steamed broccoli and cauliflower with the cheese sauce. Mix until the vegetables are well coated.
Transfer the mixture to a greased 9×13 inch baking dish or a similar-sized casserole dish, spreading it evenly.
Sprinkle breadcrumbs evenly over the top of the casserole.
Bake in the preheated oven for approximately 25-30 minutes, or until the casserole is hot and bubbly, and the breadcrumbs are golden brown. Serve hot.
