This is the best way to make them at home, the most signature part of the flavor of White Castles is actually their onion flavor and this reproduces it almost exactly.
1.5 pounds 80/20 ground beef
1 package onion soup mix
1 small onion, very finely chopped
1 dill pickle, sliced as thin as possible
mustard/ketchup (if you want to feel authentic mix them together and then spread them on the bun)
12 soft dinner rolls (I like the Sara Lee ones)
Good quality American Cheese sliced
Mix the ground beef and onion soup mix in a bowl. Spread the ground beef on a 12-15” cookie sheet. Use a rolling pin (or cover with parchment paper and flatten by hand) on the ground beef to flatten it out to about 1/4-1/2 inch thick.
Bake at 350 degrees for 10 minutes. Take out and pour off grease. Spread diced onions along the edge of the sheet so they touch the sheet where the ground meat has shrunk away from.
Bake for 15 more minutes then spread the onions evenly on top of the ground beef and layer with cheese slices. Bake another 6 minutes to melt cheese. While waiting slice your dinner rolls and wrap them all in a damp paper towel. Microwave them in the paper towel for 60 seconds to steam them. (You can also load them in a tortilla warmer instead of a paper towel if you have one.) Spread out the top buns and add mustard and ketchup on them just before you are ready to remove the ground beef from the oven.
Take it out of the oven. Spread as many pickles as you like on top of the cheese. Arrange the top buns on the ground beef to make hamburgers. Slice the ground beef between the buns with a pizza cutter and pick up the top bun and burger with a spatula and set it on the bottom bun.
Holy Crap, you just made fresh White Castles at home! Enjoy!