Preheat oven to 400°F and grease a 9x9-inch baking dish.
Blanch broccoli florets in boiling salted water for 3 minutes, then drain well and arrange in the prepared baking dish.
In a saucepan over medium heat, melt butter and sauté garlic for 1 minute, then add heavy cream, salt, pepper, and nutmeg, stirring until warmed through.
Remove cream mixture from heat and stir in half of the cheddar and Gruyere cheese until melted and smooth.
Pour the cheese sauce over the broccoli and top with remaining cheese.
Bake for 20-25 minutes until bubbling and golden brown, then broil for 2-3 minutes for extra crispiness on top.