Preheat your oven to 375°F (190°C) and generously grease a 9×13-inch (23x33cm) baking dish with nonstick spray.

In a saucepan, place the sliced Vidalia onions and add enough water to cover them. Bring to a boil, then reduce the heat and let them simmer for 8 minutes. Drain the onions and toss them with the unsalted butter.

In a large bowl, whisk together the eggs and evaporated milk until well combined. Add the cooked onions, crumbled Ritz crackers, grated cheddar cheese, salt, and pepper. Stir everything together until thoroughly mixed.

Transfer the mixture to the prepared baking dish, spreading it evenly.

For the topping, combine the crumbled Ritz crackers and melted butter in a separate bowl. Sprinkle this mixture over the casserole.

Bake in the preheated oven for approximately 45 minutes, or until the center is set and the top turns golden brown.

Once done, remove from the oven and allow the casserole to cool for a few minutes before serving.