Cut broccoli and cauliflower into large bite-sized pieces. Grease a 3-quart baking dish (9x13 inch) and add broccoli, cauliflower, and carrots.
In a medium saucepan, melt butter over medium-high heat. Add the flour and whisk until combined. Let cook for 1 minute. Add the chicken stock and whisk until thickened, about 3-5 minutes. Turn heat down to medium-low, and add the heavy whipping cream. Whisk to combine and let simmer for 2 minutes. Stir in the parmesan cheese, onion powder, garlic powder, salt, and pepper.
Pour the mixture over the vegetables in the baking dish and mix to combine. Cover with tinfoil and bake for 45 minutes until vegetables are tender. Add cheddar cheese over the top and bake uncovered for an additional 15 minutes.