1. Season beef strips with salt and pepper. In a skillet, heat oil over medium-high heat. Sear beef for about 1 minute on each side or to desired doneness. Remove strips from skillet, cover with aluminum foil, and set aside.
2. In the same skillet, sauté onion, garlic, and peppers for 4 minutes over mediumhigh heat.
3. Add balsamic vinegar and reduce until all liquid has evaporated. Add beef stock and hoisin sauce and continue cooking for 3 minutes or until sauce is slightly reduced and thickened. Add beef strips and mix.
4. Serve on a bed of rice vermicelli.

Variation:
Try this recipe in fajitas (served in tortillas and garnished with shredded lettuce).