Start my making your meat mixture.
Add the ground chuck to a large mixing bowl. Season with salt, pepper, and Lipton soup mix. To an additional bowl add the peanut butter, egg, and milk and mix until nice and smooth. To a smaller bowl add in the diced bread. Spoon in enough of the milk mixture onto the bread just until it softens up. Try not to over soak the bread as you really just want to moisten it, like a panade (trick to getting some nice moistness).
Add the milk mixture to the beef along with the bread. Mix really well.
Preheat your oven to 350 degrees.
Take a large sheet pan and line it with parchment paper. Have I told you how much I love parchment paper? Super easy cleanup. Anyway, if you don’t have parchment paper, no worries, just flatten the ground meat mixture onto the sheet pan. Go as even and as flat as you can. Remember, White Castle sliders. They will shrink and rise a bit, so no worries, but try and get to the height, if not lower than the height of the sheet tray.
Place in the preheated oven for 10 minutes, then remove from the oven.
After 10 minutes, remove the pan from the oven and carefully tilt and pour off any fat. There will be fat due to the chuck.
Take all of the onions and completely coat the meat. Place back in the oven and cook for 15 minutes.
Remove from the oven after 15 minutes, then cover the meat and onion mixture with the American cheese slices.
Place back in the oven for about 5 minutes or until the cheese begins to melt. Remove the pan from the oven, then take all of the top slider buns and place on top of the meat mixture. You can separate the buns and cut later, or separate the buns ahead of time and go that route. Whatever works for you.
Place back in the oven for a few minutes, just to warm and steam the buns through. Those Kings Hawaiian buns are soft to begin with so warming them up makes them just extra special.
Remove them after a few minutes, then using a pizza cutter, slice into columns and rows to meet the seams of the buns and use a spatula to slide each one onto a bottom bun. Plate and serve with your favorite pickles, chips, fries, whatever.