Cheesy Squash Casserole

INGREDIENTS

• 1 cup shredded sharp Cheddar
• 1/2 cup grated Parmesan
• Salt and freshly ground black pepper
• 1 tablespoon butter
• 1/2 cup sour cream
• 6 medium yellow summer squash, thinly sliced
• 1 sleeve crackers, crushed medium to fine (recommended: Ritz)
• 1 large Vidalia onion, thinly sliced
• 1 tablespoon vegetable oil