Heat olive oil in a large skillet over medium heat. Add diced onion and sauté until softened, about 3 minutes.
Stir in Worcestershire sauce, ketchup, mustard, salt, and pepper. Cook for an additional 2 minutes to blend flavors. Remove from heat.
Add shredded lettuce, tomato slices, and, if desired, pickle slices on top of the beef.
Repeat the cooking process with the remaining tortillas and beef.