Heat the oven to 375 degrees.
Add the onions to a pot, and cover in water.
Bring the water to a boil, and simmer the onions until softened, about 8 minutes.
Drain the onions and stir in the butter while they are still hot.
Beat the eggs in a large mixing bowl.
Add the onions, milk, cheese, Ritz crackers and salt and pepper to taste to the bowl with the eggs, and stir to combine well.
Spread the mixture into a well greased 9x13 baking dish.
In a bowl, stir together the topping ingredients, and sprinkle over the casserole.
Bake for 45 minutes, and allow to cool for 5 minutes before serving.