Set an oven rack in the center of your oven and preheat it to 375°F. Line a sheet pan with a sheet pan rack. Add the ground beef and pork to a bowl. Sprinkle with onions, salt, and burger seasoning, and lightly mix.
Add a sheet of parchment paper to your work surface. Press the meat onto the parchment paper and shape it into a 8x12-inch rectangle. Place the parchment paper with the meat on it on the sheet pan rack.
Top the meat with another sheet of parchment paper, then place a second sheet pan on top of it. Bake for 20 minutes or until the internal temperature of the meat reaches 150°F.
In the meantime, slice the rolls in half lengthwise. Brush the rolls with 2 tablespoons of butter.
Once the 20 minutes are up and the meat has reached an internal temperature of 150°F, remove the top sheet pan from the oven. Then remove the sheet pan that has the meat in it from the oven. Remove the top layer of parchment paper to reveal a large meat patty.
Use two large spatulas to remove the meat from the parchment paper on the sheet pan rack. Top the bottom rolls with the large meat patty.
Add the 6 slices of American cheese in a single layer.
Place the top rolls on top of the meat and cheese. Brush the tops with the last 2 tablespoons of butter. Sprinkle with 2 teaspoons of Everything But The Bagel Seasoning.
Cook for 15 minutes or until the bread toasts and the cheese melts. Remove from the oven and cool for a few minutes before slicing into individual sliders. Serve on a platter and enjoy your the oohs and ahhs at your get-together!