Preheat your oven to 375°F (190°C).
Steam or blanch broccoli florets until they're just tender. Drain well.
In a saucepan over medium heat, melt the cream cheese. Add heavy cream, 1 1/2 cups of cheddar cheese, minced garlic, diced onion, salt, and black pepper. Stir constantly until the mixture is smooth and creamy.
In a large mixing bowl, combine the drained broccoli and cheese sauce. Mix well, ensuring broccoli is well-coated.
Pour the broccoli-cheese mixture into a greased baking dish.
Sprinkle the remaining 1/2 cup cheddar cheese and 1/4 cup grated Parmesan on top.
Place the baking dish in the oven and bake for 20-25 minutes, or until the top is golden and bubbly.
Allow to cool slightly before serving.