Merritt's Butternut Squash Gratin

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
• 1/4 cup grated Parmesan cheese