Heat olive oil on a large skillet over medium heat. Add diced onion and cook until caramelize (5-10 minutes). Add red pepper and cook until translucent (about 5 minutes). Once cooked, remove from heat and set aside.
Mix taco seasoning with ground beef until well coated. In the same skillet, heat ground beef over medium-high until cooked through (about 10 minutes or until there is no pink). Remove from heat and let cool for 2 minutes.
Put it all together: Plate corn tortilla shells. Cover with half a handful of lettuce. Spoon over cooked beef, red pepper and onion mixture, and top with cheese. Serve with lime wedges and enjoy.