Garlic and Herb Butternut Squash Gratin

Name: Garlic and Herb Butternut Squash Gratin
Cuisine: European, Mediterranean
Ingredients: 4 ounces shredded Gruyere cheese, divided; 2 ounces grated Parmesan cheese, divided; 2 cups heavy cream; 5 cloves garlic, minced; 2 teaspoons fresh thyme leaves, chopped; 1 teaspoon fresh sage, chopped, plus whole leaves to top; 1 teaspoon fresh rosemary, chopped; Kosher salt and black pepper, to taste; 1 tablespoon butter; 1 large (3-4 pound) butternut squash, peeled and sliced into thin half-moons; coarse finishing salt, if desired, to top