Start by preheating your oven to 375°F. Grease a 9x13-inch baking dish with nonstick spray.
Put your onion slices in a saucepan, add just enough water to cover them, and bring it to a boil. Reduce the heat and let it simmer for 8 minutes. Drain and mix the onions with butter.
In a sizable bowl, whisk the eggs and evaporated milk together. Add in the buttered onions, cracker crumbs, cheddar cheese, salt, and pepper. Stir it all up.
Pour the mixture into the greased baking dish. For the topping, combine the melted butter and cracker crumbs. Sprinkle it over your casserole.
Bake until the center sets and the top turns a delicious golden brown, about 45 minutes.