Instructions Preheat the oven to 375°F and grease a 9×13-inch baking dish with nonstick spray.Place onions in a saucepan and add water just to cover. Bring to a boil, reduce heat and let simmer for 8 minutes. Drain and toss onion slices with butter.In a large bowl, whisk together eggs and evaporated milk. Add cooked onions, cracker crumbs, cheese, salt, and pepper. Stir to mix.Transfer to baking dish. Toss together cracker crumbs and melted butter for the topping. Sprinkle over casserole.Bake until set in the center and top is golden brown, about 45 minutes. Enjoy!