Cheesy Sage and Butternut Squash Gratin

Name: Cheesy Sage and Butternut Squash Gratin
Prep time: 10 min
Cook time: 1 h
Total time: 1 h 10 min
Cuisine: European, Mediterranean, French
Ingredients: 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