Form the ground turkey into patties. Season with salt and pepper and set aside. In a skillet with a little water, cook the onion rings until soft and caramelized. Add more water a little at a time as needed during the cooking process. Remove from skillet and set aside. Spray the skillet with zero calorie cooking spray. Cook turkey burgers over medium high heat, adding a little water as needed, until the turkey is cooked through (Internal temperature should be 165 degrees.) Make the sauce. In a bowl, add the ingredients for the sauce and whisk to combine. Place the turkey burger on the butter lettuce leaves. Slather with sauce and top with onions. Serve with slices of tomatoes on the side.