Two-Cheese Squash Casserole

INGREDIENTS

• 4 pounds yellow squash, sliced
• 4 tablespoons butter or margarine, divided
• 1 large sweet onion, finely chopped
• 2 garlic cloves, minced
• 2 1/2 cups soft breadcrumbs, divided
• 1 1/4 cups shredded Parmesan cheese, divided
• 1 cup (4 ounces) shredded Cheddar cheese
• 1/2 cup chopped fresh chives
• 1/2 cup minced fresh parsley
• 1 (8-ounce) container sour cream
• 1 teaspoon salt
• 1 teaspoon freshly ground pepper
• 2 large eggs, lightly beaten
• 1/4 teaspoon garlic salt