01 - Preheat oven to 350°F (180°C). Lightly grease a medium-sized baking dish with butter or nonstick spray.
02 - Steam or blanch broccoli florets and carrot slices for 3 to 4 minutes until just tender. Drain thoroughly and set aside.
03 - In a medium saucepan over medium heat, melt 2 tablespoons butter. Whisk in flour and cook, stirring, for 1 minute. Gradually pour in milk, whisking continuously until the sauce thickens and becomes smooth.
04 - Remove pan from heat. Stir in shredded cheddar cheese, garlic powder, onion powder, salt, and pepper. Mix until cheese is fully melted and the sauce is creamy.
05 - Gently fold steamed broccoli and carrots into the cheese sauce until evenly coated. Transfer the mixture to the prepared baking dish, spreading out evenly.
06 - In a small bowl, mix panko breadcrumbs with melted butter and grated Parmesan cheese, if using. Sprinkle the breadcrumb mixture evenly across the top of the casserole.
07 - Bake for 25 to 30 minutes, until the top is golden brown and bubbling. Allow to rest for 5 minutes before serving.