Preheat oven to 350 degrees Fahrenheit. Spray a 9×13-inch oven-safe dish or pan with cooking spray.

Place frozen broccoli and cauliflower in dish.

In a medium saucepan, melt three tablespoons butter over medium heat.

Add flour and mix until you get a thick paste.

Add one cup milk and whisk until thickened.

Add another cup milk and continue mixing until it thickens.

Add salt, pepper, paprika, garlic powder, and onion powder. Mix until combined and remove from heat.

Add 3/4 cup mozzarella cheese and stir until combined.

Pour cheese sauce over broccoli and cauliflower and mix until coated.

Sprinkle remaining cheese over vegetables. Sprinkle panko crumbs over cheese.

Bake for 50–60 minutes or until top is golden.