Preheat the oven to 375°F (190°C). Grease a 9×13-inch baking dish.
Prepare the vegetables:
Bring a pot of salted water to a boil and blanch the broccoli and cauliflower for 3-4 minutes until they are just tender. Drain and set aside.
Mix the sauce:
In a large mixing bowl, combine the sour cream, cream cheese, mayonnaise, butter, garlic powder, onion powder, and half of the cheddar cheese. Mix until smooth.
Season with salt and pepper to taste.
Combine the casserole:
Add the drained broccoli and cauliflower to the sauce mixture, stirring until the veggies are well coated.
Assemble the casserole:
Transfer the mixture to the greased baking dish. Top with the remaining cheddar cheese and Parmesan cheese.
(Optional) Sprinkle bread crumbs on top for a crispy finish.
Bake:
Bake in the preheated oven for 20-25 minutes or until the cheese is bubbly and the top is golden brown.
Serve:
Remove from the oven, sprinkle with paprika if desired, and let cool for a few minutes before serving.