Shape the beef into 6 burger patties and generously season with salt and pepper.
Place the burger patties directly on the grill. Close the lid and cook for 4 minutes, then flip and cook for an additional 4 minutes for medium. Place a slice of cheese on top of each burger during the last minute of cooking.
Remove the burgers from grill and let rest for 2 minutes.
Place the burgers on buns and top with butter lettuce, tomato slices, and red onion slices, along with any desired condiments.