Start by preparing the ground beef. Divide the 1 lb of 80/20 ground beef into 6 equal portions. Roll each portion into a ball and set aside.
Next, slice the 1/2 large sweet onion into 1/4 inch rounds and slice the 1 jalapeño pepper. Set them aside for later use.
Heat a griddle over medium heat. Once it's hot, add the onions and jalapeños to the griddle. Cook for about 5-7 minutes, or until the onions and peppers are nice and tender. Remove them from the griddle and set them aside.
Now increase the heat of the griddle to high and add the meatballs. Use a burger smasher or 2 spatulas to press them down, flattening the meatballs into thin patties. Cook them for about 3-4 minutes on one side, or until the tops become watery.
Once the meat patties are cooked, add a slice of onion and a few slices of jalapeño to each patty. Place a slice of American cheese on top of the veggies. Place another patty on top of the cheese, creating a sandwich. Cook for another 2-3 minutes, or until the cheese is fully melted.
While the patties are cooking, place the 3 burger buns on the griddle, cut side down. Toast them for about 1-2 minutes or until they are golden brown. Once they are toasted, remove them from the griddle and top them with a slice of American cheese.
Once the cheese is fully melted, remove the patties from the griddle. Assemble the burgers by placing the patties on the bottom buns, then add the cooked onions and jalapeños on top. Add the top buns and serve. Enjoy your Grilled Cheese Cheeseburgers!