Commence by preheating the oven to 375°F. Simultaneously, prepare a 9x13-inch baking dish by greasing it thoroughly with a nonstick spray.

In a medium-sized saucepan, submerge the sliced Vidalia onions with water. Set the heat to high, bringing the water to a boil. Upon reaching a boil, lower the heat, allowing the onions to simmer for approximately 8 minutes. Following this, ensure the onions are adequately drained, then proceed to mix them with the unsalted butter.

In a separate, larger bowl, vigorously whisk the eggs and evaporated milk until thoroughly combined. Incorporate the previously prepared onions, followed by the Ritz cracker crumbs, grated cheddar cheese, salt, and pepper. Ensure the mixture is well-integrated.

Transfer the harmonized mixture into the baking dish. For the topping, meticulously combine the Ritz cracker crumbs with the melted butter. Sprinkle this combination uniformly over the casserole.

Place the dish into the preheated oven, allowing it to bake until the center is firm and the top has achieved a golden brown hue, which typically requires 45 minutes. Serve upon completion.