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 to remove excess moisture.
03 - In a saucepan over medium heat, melt the butter. Add the heavy cream and cream cheese, whisking constantly 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 continuously, 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 top is golden brown and the sauce is bubbly around the edges.
08 - Let the dish cool for 5 minutes before serving to allow the sauce to set slightly. Garnish with chopped fresh parsley if desired.