Mama's Summer Squash Casserole

Ingredients (6)

• + 8 yellow summer squash, thinly sliced add to pantry
• + 1 small yellow onion, thinly sliced add to pantry
• + 1 egg, lightly beaten add to pantry
• + 1 cup low-fat sour cream add to pantry
• + 1 sleeve buttery round crackers (such as Ritz®), crushed, divided add to pantry
• ✓ 2 cups shredded sharp Cheddar cheese, divided