01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or cooking spray.
02 - Bring a large pot of salted water to a boil. Add broccoli and cauliflower florets, blanch for 3–4 minutes until just tender. Drain well and pat dry thoroughly.
03 - In a saucepan over medium heat, melt the butter. Add the heavy cream and cream cheese, whisking until smooth and slightly thickened, about 2–3 minutes.
04 - Stir in 1 cup of cheddar cheese, Parmesan cheese, garlic powder, onion powder, paprika, salt, pepper, and nutmeg. Cook, stirring constantly, until cheeses are completely melted and sauce is creamy and smooth. Remove from heat.
05 - Spread the blanched broccoli and cauliflower evenly in the prepared baking dish. Pour the cheese sauce over the vegetables and gently toss to coat evenly.
06 - Sprinkle the remaining ½ cup cheddar cheese evenly over the top of the vegetable mixture.
07 - Bake for 20–25 minutes, until the cheese is golden brown and bubbly around the edges.
08 - Let the casserole cool for 5 minutes before serving. Garnish with chopped fresh parsley if desired.