Slice 8 large rounds from the edges of the head of iceberg to create buns.
Add onion slices to a skillet and cook until tender, about 3 minutes per side. Set aside red onions and wipe the skillet clean.
Return skillet over medium-high heat. Shape ground turkey into 4 large burger patties. Season both sides of patties with pepper and add to skillet. Cook until seared on both sides and cooked to your liking, about 4 minutes per side, but make sure it’s not pink inside. I use a meat thermometer and wait until it reaches 165 degrees.
Top each burger with a slice of cheese, then cover skillet with a lid and cook until cheese has melted, about 1 minute more.
To build each burger, top one iceberg round with the cooked turkey cheeseburger, avocado slices, and a tomato slice. Top with second iceberg round. Repeat with remaining ingredients and serve.