In a small skillet, cook beef over medium heat until no longer pink, 4-6 minutes, breaking it into crumbles; drain. Stir in pine nuts, salt and pepper; cool slightly. Stir in 2 tablespoons yogurt., Arrange pita chips on a serving platter. Layer with beef mixture, tabbouleh, hummus, tomato, olives, parsley, mint, remaining yogurt and, if desired, onion. Serve immediately.