In large bowl, combine sour cream, soup mix and egg; pepper to taste. Add turkey and bread crumbs; mix until well combined. Form into 8 patties.
Prepare grill to medium heat. Grill burgers, turning once, until internal temperature reaches 170°F (about 8-10 minutes). Top with cheese.
Place burgers on buns; top with grilled onions, tomato, avocado and lettuce.