For rub, combine chilli powder, garlic salt, cumin, oregano and ground red pepper in small bowl; mix well. Transfer to container with tight-fitting lid. Store in cool dry place up to 2 months.
Brown beef and onion in large deep frying pan over medium-high heat, stirring to separate meat. Drain and discard fat. Sprinkle chili rub over beef mixture; cook 1 minute. Reduce heat to medium. Add water and tomato paste. Cover; simmer 5 minutes.
Spoon beef mixture into taco shells; top with cheese. Arrange lettuce, tomatoes avocado, sour cream and salsa in bowls. Serve tacos with toppings.