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

Remove the skillet from heat and let it cool slightly. Stir in heavy cream and beaten eggs until combined. Fold in half of the sharp cheddar cheese.

Grease a 9x13-inch baking dish and transfer the onion mixture into it, spreading evenly. Sprinkle remaining cheese over the top.

Mix crushed butter crackers with melted butter and distribute them over the casserole. Bake uncovered for 30 to 35 minutes until golden brown and bubbly.

Let the casserole rest for about 5 minutes before serving. Serve it alongside your favorite roasted meats.