Slice buns in half. Warm a large skillet over medium heat, and then add 1 tablespoon of the butter. Once butter has melted, add buns, cut side down, and toast until they are golden brown. If needed, do this in batches, adding a tablespoon of butter to the skillet with each new batch. Remove buns from skillet to a plate and set aside.

Heat same skillet over medium-low heat with the remaining tablespoon of butter and stir in the onion. Cook for 6-8 minutes, stirring occasionally, until onions are softened and translucent. Remove cooked onions from skillet and set aside.

Heat same skillet over medium heat. Season ground beef with salt, pepper, and garlic powder. Shape ground beef into a large patty form, about 1" thick, and add it to the heated skillet. Cook until browned on the bottom side, about 5-6 minutes. At this point, if desired, drain excess fat from the pan. Then flip the patty over and cook for another 4-5 minutes, with some pink color still in the center of the patty. Cook time will depend on thickness of ground beef patty and cooking temperature.

Add the onions back to the skillet, with the cooked burger patty. With a rigid spatula, "chop" the beef patty and onions together, until you have bite-size pieces of beef. Once the pink color is gone from the beef, divide the chopped mixture into four equal portions, right in the skillet.

Place 1-2 slices of cheese on top of each portion and cook for 1-2 minutes, until cheese is melted and gooey.

To assemble sandwiches, first spread mayo on one side of the toasted bun and ketchup on the other, if desired. Then transfer each portion of meat and cheese mixture onto individual hoagie buns. Top with lettuce and tomatoes, plus any other condiments that you like. Enjoy immediately, while hot.