Remove the stalk and seeds from the peppers and cut into small dice.
Slice the celery finely.
Grate the cheese.
Place the beef slices, peppers, celery, cheese, mayonnaise and yogurt in a bowl. Mix together.
Heat the tortillas for a few seconds in the microwave.
Assemble the tortillas: place half the beef mixture in the middle of the bottom half of 1 tortilla, fold the tortilla ensuring all the filling is covered.