In a large bowl, combine beef, salt and pepper, mixing lightly but thoroughly. Shape into eight 1/2-in.-thick patties., Grill burgers, covered, over medium heat or broil 3-4 in. from heat until a thermometer reads 160°, 3-4 minutes on each side. Place burgers in lettuce leaves. Combine feta and Miracle Whip; spread over burgers. Top with avocado, red onion and, if desired, tomatoes.