Combine Lipton onion soup mix, ground beef, water, and egg yolk in a large bowl. Mix gently until well combined. (I like to use clean hands, but you can use a spoon or potato masher.)
Divide the meat mixture into four hamburger patties. Make a divot in the middle of each patty to prevent the edges from curling.
Heat a skillet to medium or medium-high heat. Cook the burgers for 3-5 minutes per side.
If desired, top each patty with cheese for the last minute of cooking, and cover the skillet with a lid.
Spread mayo and ketchup on the top bun, then add the lettuce, tomato, and hamburger patty. Serve immediately.
Notes
Cooking time will vary depending on the thickness of your burger.
For extra tasty burgers, spread butter on the buns and toast them in a skillet or under a broiler.