Place all ingredients in a large bowl and mix by hand until just combined. Divide the mixture into four patties. Use your thumb to make an indentation in the center of each patty—this helps the burgers stay flatter and cook more evenly.
Preheat a grill to medium high. Add the burgers and cook for about five minutes, then flip and top with cheese, if using. Cook for 2-5 minutes on the second side, or until the cheese is melted and the burgers are cooked to your desired doneness.
Put a piece of lettuce on the bottom half of each bun. Top with a cheeseburger and then add sliced onion, tomato, and avocado. Serve hot.
Notes
Instead of grilling, you can also cook the burgers in a grill pan or cast iron skillet. Cooking time and temperature will be similar.
If you’d like to simplify the recipe and have a packet of onion dip or soup mix on hand, you can use that instead of the seasonings. Simply add one packet to the beef, egg, and breadcrumbs.