Preheat your oven to 350°F (175°C).

Melt unsalted butter in a large skillet over medium heat. Add sliced sweet onions, salt, and black pepper. Cook for about 15 minutes until soft and golden.

Remove skillet from heat and allow onions to cool. In a mixing bowl, whisk together sour cream and beaten eggs until smooth. Combine with sautéed onions and fold in half the sharp cheddar and mozzarella cheeses.

Lightly grease a 9x13-inch baking dish. Transfer the onion filling into the dish, spreading evenly. Sprinkle remaining cheese on top.

In a small bowl, combine crushed butter crackers with melted butter, mixing until coated. Sprinkle over the casserole.

Bake uncovered for 30–35 minutes until the top is golden brown and filling is bubbling. Allow to rest for about 5 minutes before serving.