Preheat your oven to 350°F (175°C). In a large skillet, melt 4 tablespoons of unsalted butter over medium heat. Add the 4 thinly sliced sweet onions, seasoning them with 1 teaspoon of salt, ½ teaspoon of black pepper, and the optional teaspoon of sugar. Cook for about 15 minutes until the onions are soft and lightly golden.

Remove the skillet from heat. In a large mixing bowl, combine the cooked onions with 1 cup of sour cream and 2 lightly beaten eggs, stirring until the mixture is smooth. Fold in 1 cup of shredded sharp cheddar cheese and ½ cup of shredded mozzarella cheese.

Grease a 9x13-inch baking dish. Pour the onion and cheese filling into the dish, spreading it out evenly. Sprinkle any remaining shredded cheese on top.

In a small bowl, combine 1 cup of crushed butter crackers with 2 tablespoons of melted butter. Stir until the crackers are evenly coated. Sprinkle this mixture over the casserole.

Place the casserole in the preheated oven and bake uncovered for 30–35 minutes until the top is golden brown and the edges are bubbling.

Let the casserole sit for about 5 minutes before serving.