1. To make the sauce, in a small bowl, combine yogurt, mustard, and relish. Mix well.
2. In a medium bowl, combine turkey, onion, egg whites/substitute, garlic powder, and pepper. Evenly form into a 4-inch wide patty.
3. Bring a grill pan (or skillet) sprayed with nonstick spray to medium-high heat. Cook patty for 5 minutes per side, or until cooked through.
4. Meanwhile, combine black beans with salsa/pico de gallo in a small microwave-safe bowl. Cover and microwave for 1 minute, or until hot. Mix well.
5. Stack two large lettuce leaves, and place tomato in the center (or stack two extra-large leaves, and place tomato over one half). Top with turkey patty and sauce, and finish with the remaining two large lettuce leaves (or wrap the extra-large leaves around the filling).
6. Serve with black beans.