Chop the onion. Heat a few tablespoons of oil in a pan over medium-high heat, add the chopped onion, and fry for 3-4 minutes until translucent. Add the minced round beef, season with salt and pepper, and cook for 5-7 minutes, stirring occasionally, until cooked through. Drain off any excess liquid.
Add the ketchup, mustard, garlic powder, and Worcestershire sauce to the pan. Stir for about 2 minutes until the mixture is well combined.
Melt 1 tablespoon of butter in a separate pan over medium heat. Add one tortilla, place some of the meat mixture on one half, sprinkle with grated cheese, and fold the tortilla in half. Fry for 3-5 minutes, turning once halfway through, until both sides are golden brown. Repeat with the remaining tortillas, melting 1 tablespoon of butter for each.
Serve the tortillas on a plate and garnish with additional ketchup and/or mustard if desired. Serve with a mixed salad on the side.