1 large onion, peeled, halved, and thinly sliced
Salt and pepper, to taste
1 lb ground beef
1 tablespoon butter
1 tablespoon vegetable oil
4-8 slices American cheese
Mayo, mustard, and pickles, for serving
4 hamburger buns, toasted
In a bowl, combine the sliced onions with 1 teaspoon of salt, tossing to mix. Transfer the onions to a colander and let them sit for 30 minutes, tossing occasionally. Afterward, place the onions in a clean dish towel, gather the edges, and squeeze out excess liquid over the sink or a large bowl.
Divide the drained onions into four separate mounds on a rimmed baking sheet. Shape the ground beef into four lightly packed balls and place them on top of the onion mounds. Flatten the beef firmly with your palm so that the onions adhere to the beef, forming patties that are 4 inches in diameter. Season generously with salt and pepper.
In a 12-inch skillet over medium heat, melt the butter with the vegetable oil. Using a large spatula, transfer the patties to the skillet with the onion side down. Cook for 6-8 minutes until the onions become a deep golden brown and start to crisp around the edges. Flip the burgers, increase the heat to high, and cook until well-browned on the second side, approximately 2 minutes.
Place 1 or 2 slices of American cheese on each burger and allow it to melt. Spread mayo, mustard, and add pickles to the top buns. Then, assemble each burger on the bottom buns. Serve immediately.