Mena's Baked Macaroni and Cheese with Caramelized Onion

INGREDIENTS

• 6 tablespoons butter, divided
• 1 large Vidalia or other sweet onion, thinly sliced
• 1/2 teaspoon sugar
• 1 pinch salt
• 1 (8 ounce) box uncooked macaroni
• 1 cup plain bread crumbs
• 1/2 teaspoon dried mustard
• 1 clove garlic, minced
• 1 pinch cayenne pepper
• 3 tablespoons all-purpose flour
• 1 3/4 cups milk
• 3/4 cup low-sodium chicken broth
• 8 ounces grated sharp white Cheddar cheese
• 1/2 cup grated Parmesan cheese
• salt and black pepper to taste