Preheat oven to 180°C/160°C fan-forced.
Heat oil in a large frying pan over medium-high heat. Add onion. Cook, stirring, for 3 minutes or until softened. Add cumin, coriander, paprika and cayenne pepper. Cook, stirring, for 30 seconds or until fragrant. Add mince. Cook, stirring with a wooden spoon to break up mince, for 8 minutes or until browned. Add crushed tomato. Cook, stirring occasionally, for 5 minutes or until sauce has thickened.
Meanwhile, heat taco shells following packet directions. Divide beef mixture and salad tacos evenly between taco shells. Top with lettuce, chopped tomato and cheese. Serve.