Slice off root and stem end of onions; remove outer skins. Place onion halves on cutting board with cut sides down and slice onion halves into slivers about 1/4-inch to 1/2-inch thick.
Place onions in a saucepan and add water to just cover. Bring to a boil; reduce heat and simmer 8 minutes. Drain well.
Toss warm onions with the 2 tablespoons butter.
While onions cook, beat the eggs and evaporated milk together in a large bowl.
Add the warm cooked onions, cracker crumbs, shredded cheese, salt, and pepper. Stir to combine.
Place onion mixture in a 9x13-inch baking dish coated with non-stick cooking spray.
Prepare Crumb Topping:
Combine cracker crumbs and melted butter.
Sprinkle evenly over the top of the casserole.
Bake: Bake at 375 degrees for 45 minutes, until set in the middle and golden brown.