Preheat the oven to 375°F and grease a 9×13-inch baking dish.
Place the sliced onions in a pot of water, bring to a boil, and simmer for about 8 minutes, until softened. Drain and stir in 2 tbsp butter while hot.
In a large bowl, beat the eggs. Add the cooked onions, shredded cheese, crushed Ritz crackers, evaporated milk, salt, and pepper. Mix until combined.
Spread the mixture evenly in the prepared baking dish.
For the topping, combine the crushed Ritz crackers with 3 tbsp melted butter, then sprinkle evenly over the casserole.
Bake for 45 minutes, until the top is golden and bubbly. Let cool for 5 minutes before serving.
Enjoy!