Heat a large skillet over medium-high heat. Add oil; swirl to coat. Add onion; saute 4 minutes, stirring occasionally. Add garlic and the next 6 ingredients (through black pepper); saute 1 minute, stirring constantly. Add beef; cook 3 minutes or until browned, stirring to crumbled. Drain oil from pan. Stir in tomato sauce. Cover, reduce heat, and simmer 10 minutes, stirring occasionally.
Spread 2 tablespoons beans evenly over 1 side of each flour tortilla; sprinkle each with about 2 tablespoons cheese. Place tortillas on a baking sheet; broil 2 minutes or until cheese melts. Carefully wrap 1 tortilla around 1 hard taco shell, pressing gently until cheese sticks to the shell. Repeat procedure with remaining tortillas and shells.
Spoon about 1/4 cup beef mixture into each shell; top each with 2 tablespoons lettuce, 2 tablespoons tomato, 1 tablespoon sour cream, and a sprinkle of avocado.