In a large skillet, cook beef and onion over medium heat until no longer pink; drain. Add the next 12 ingredients to meat mixture. Simmer, uncovered, for 10-15 minutes or until liquid is almost completely reduced, stirring occasionally. Cool slightly; stir in cheese. , Place taco shells open end up in a baking pan; place a scoop of meat mixture into each shell. Bake at 400° for 10-15 minutes until meat is hot and cheese is melted. , Sprinkle lettuce and tomatoes over tacos. In a small bowl, combine dressing ingredients; drizzle over tacos.