Preheat your oven to 350°F (175°C). In a large skillet, melt 2 tablespoons of unsalted butter over medium heat. Add thinly sliced sweet onions, along with salt, black pepper, and optional sugar. Sauté for about 15 minutes until the onions are soft and lightly golden.

Remove the skillet from heat and add in 1 cup of sour cream and 2 beaten eggs. Stir until smooth, then gently fold in 1 cup of sharp cheddar and 1 cup of mozzarella cheese.

Grease a 9x13-inch baking dish. Transfer the onion mixture into the dish, spreading evenly. Sprinkle remaining cheeses on top. In a separate bowl, mix crushed butter crackers with 2 tablespoons of melted butter and spread over the casserole.

Place the casserole in the oven and bake uncovered for 30-35 minutes until bubbling and golden on top.

Allow to sit for 5 minutes before slicing and serving alongside your favorite roasted meats.