Roasted Butternut Squash Lasagna

Name: Roasted Butternut Squash Lasagna
Ingredients: 1 butternut squash, peeled and cut into 1/2-inch cubes; 2 tablespoons extra-virgin olive oil; Kosher salt; 1 onion, halved and sliced; 2 tablespoons unsalted butter; 10 sage leaves; 1/4 cup all-purpose flour; 4 cups whole milk; Pinch of freshly grated nutmeg; Unsalted butter, for the baking dish; 1 cup grated fontina cheese (about 4 ounces); 1 cup grated low-moisture mozzarella cheese (about 4 ounces); 1 cup grated parmesan cheese (about 4 ounces); 1 tablespoon all-purpose flour; 9 no-boil lasagna noodles