Merritt's Butternut Squash Gratin

Name: Merritt's Butternut Squash Gratin
Prep time: 40 min
Cook time: 1 h 20 min
Total time: 2 h
Cuisine: American
Ingredients: 3 tablespoons butter; 1 yellow onion, diced; 2 cloves garlic, minced; 1 (2 1/2 pound) butternut squash - peeled, seeded, and cut into 3/4-inch chunks; 1 teaspoon brown sugar; 1 cup chicken broth; 8 ounces shredded Gruyere cheese; 8 ounces shredded extra-sharp Cheddar cheese; 1 cup dry bread crumbs; 1 tablespoon chopped fresh thyme; 1 tablespoon chopped fresh rosemary; 0.25 cup grated Parmesan cheese