1. Trim fat from beef, slice into ¼ inch strips. Set aside.
2. Combine the mustard, pepper, garlic and salt in a large bowl.
Add beef, tossing to coat.
3. Place a large skillet coated with spray over medium high heat until
hot. Add beef mixture; sauté 4 minutes or until done.
Remove from pan; keep warm. Add mushrooms, onion, and bell pepper
to pan; sauté 2 minutes. Add water; cover and cook 5 minutes,
stirring occasionally. Stir in beef mixture and vinegar. Remove
from heat; stir in cheese.
Warm tortillas and spoon in.