01 - Preheat oven to 400°F (200°C). Lightly grease a medium baking dish, approximately 8x12 inches.
02 - Boil gnocchi in salted water until they float, typically 2-3 minutes. Drain and set aside.
03 - Melt butter in a large skillet over medium heat. Add sage leaves and cook, swirling, until butter turns golden brown and fragrant, and sage leaves are crisp, about 4 minutes. Remove sage leaves with a slotted spoon and reserve. Remove skillet from heat.
04 - To the browned butter, add minced garlic and cook for 30 seconds until fragrant. Stir in heavy cream, salt, pepper, and nutmeg (if using). Simmer gently for 2 minutes.
05 - Add cooked gnocchi to the skillet and gently toss to coat. Stir in half of the Parmesan and half of the mozzarella.
06 - Transfer the mixture to the prepared baking dish. Sprinkle with remaining Parmesan and mozzarella. Top with crispy sage leaves. Bake for 15 minutes, until cheese is melted and bubbly and the top is lightly golden. Let rest for 5 minutes before serving.