Combine all of the sauce ingredients except for the lettuce in a small bowl.
Add in 1/4 cup of the shredded lettuce and stir to coat.
Reserve until ready to use.
For the burgers
Preheat a griddle on your grill on high for 10 minutes.
Form the beef into 4 4-ounce balls.
Place the balls onto the griddle and let cook for 30 seconds then flatten them out until then using a spatula or grill press. Season well with the salt and pepper.
After 3 minutes flip the patties and season. Continue cooking until almost done to your preference.
Add the cheese and cook another minute until melted and the burgers are done.
While the burgers are cooking toast the buns.
Remove burgers from the griddle and place onto the bun bottoms. Top with the lettuce, plenty of the sauce mixture, and onion slices.