1. To prepare onions, heat 1 teaspoon oil in a large nonstick skillet over medium-high heat. Add sliced onion to pan; sauté 12 minutes or until browned, stirring occasionally. Stir in vinegar, sugar, and 1/8 teaspoon salt; cook 30 seconds or until vinegar is absorbed. Remove onion mixture from pan; keep warm. Wipe pan dry with a paper towel.
2. To prepare burgers, place potato in a large saucepan; cover with water. Bring to a boil. Reduce heat, and simmer 15 minutes or until tender; drain.
3. Heat large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add chopped onion and garlic to pan; sauté 5 minutes or until tender.
4. Place potato, chopped onion mixture, oats, cumin, 3/4 teaspoon salt, and pepper in a food processor; process until smooth. Place potato mixture in a large bowl; stir in nuts. Divide potato mixture into 6 equal portions, shaping each into a 1/2-inch-thick patty.
5. Wipe pan dry with a paper towel. Heat 1 1/2 teaspoons oil in pan over medium-high heat. Add 3 patties to pan; cook 4 minutes or until browned. Carefully turn patties over; cook 3 minutes or until browned. Remove from pan; keep warm. Repeat procedure with remaining 1 1/2 teaspoons oil and 3 patties. Place lettuce leaves and patties on bottom halves of buns; top each patty with 1 tablespoon chili sauce, about 3 tablespoons onion, and top halves of buns.