Mix soy sauce, sugar, Mirin, water and cornstarch well in a pot. Cook at medium heat until boiling, and then let it boil for 1 minute. It becomes a very thick Teriyaki sauce.

Mix ground beef, salt and pepper. Divide into 4 parts and make into 4" round patties.

Heat a skillet at medium high heat, add oil, and place the patties in the skillet when it gets hot. Cook the patties for 2-3 minutes on each side (or until cooked to your taste).

On the bottom half of the hamburger buns put the cooked meat. Then pour Teriyaki sauce over the meat. Add the sliced tomato and onions and the shredded lettuce. Drizzle mayo on the lettuce and top it with the top half of the buns.