Brown meat and chopped onion in large skillet, stirring til meat crumbles; drain well. Add tomato sauce and taco seasoning to meat mixture. Stir well. Bring to boil. Reduce heat and simmer, uncovered for 5 minutes.
Split and toast hamburger buns. Place onion rings on each sandwich; top with meat mixture. Top each sandwich with lettuce, tomato, cheese and olives. Replace bun tops. Makes 6 sandwiches.