1 large (2-3 pounds) butternut squash, peeled, halved, seeded, and thinly sliced into 1/8” half-rounds
2 tablespoons unsalted butter, plus more for baking dish
1 large yellow onion, thinly sliced
1 garlic clove, finely chopped
2 tablespoons fresh sage, thinly sliced
1/4 teaspoon grated nutmeg
Kosher salt and freshly ground black pepper
1 cup heavy cream
1/4 cup fresh bread crumbs
1 cup Gruyère cheese, shredded, divided
01Preheat oven to 400 F. Lightly grease large round or rectangular baking dish with butter. Layer half of squash slices in rows or concentric circles in baking dish and set aside.
02Melt butter in large skillet over medium heat. Add onion and cook until tender, 5-7 minutes. Stir in garlic and sage and cook additional 3 minutes. Season with nutmeg and salt and pepper to taste. Whisk in heavy cream and bring to simmer. Remove from heat and stir in 3/4 cup Gruyère until smooth.
03Pour half of cheese sauce over layered squash and top with another layer of remaining squash. Pour over remaining cheese sauce. Cover tightly with foil and bake 30 minutes.
04Remove foil and top with bread crumbs and remaining cheese. Bake additional 15-20 minutes until squash is tender and cheese is browned and bubbly.