Begin by slicing the sweet onions thinly. In a skillet, melt 2 tablespoons of unsalted butter over medium heat. Add onions and sauté until soft and lightly golden.

In a mixing bowl, combine the sautéed onions with sour cream, beaten eggs, salt, black pepper, and half of the shredded cheeses. Mix until smooth.

Grease a 9x13-inch baking dish with butter. Pour the onion filling into the dish, spreading it evenly. Sprinkle remaining cheeses and crushed butter crackers on top, followed by melted butter.

Preheat your oven to 350°F (175°C). Bake the casserole for 30-35 minutes until golden brown on top and bubbling around edges.

Remove casserole from the oven and let it rest for 5-10 minutes before slicing and serving.