2-3poundssirloin roast, cut into 1-inch cubes (or same amount of chicken breasts, cubed)
1red onion, cut into squares (optional)
In a small bowl, whisk together the vinegar, oil, mustard, garlic, salt, pepper and red pepper flakes. Measure out 1/3 cup of the mixture and set aside for basting later.
Add the rosemary to the remaining marinade. Place the beef or chicken in a large, gallon-size ziploc bag and pour in the marinade. Seal and refrigerate for 4-8 hours (or as little as 20 minutes, although the flavor won't be quite as pronounced).
Preheat a grill to medium heat.
If using wooden or bamboo skewers, soak in cool water for 15-20 minutes. Thread red onion onto the skewer followed by meat and then a few red onions to finish off.
Whisk the reserved basting sauce to recombine. Grill the kebabs, turning every couple of minutes and basting with the sauce, until the beef is cooked to desired doneness (chicken should be cooked all the way through), 10-12 minutes.
Transfer the kebabs to a platter or plate and tent with foil to let rest for 5 or so minutes.
Season with salt and pepper to taste and serve.
Vinegar:be aware that because of the balsamic vinegar, if using chicken, the light-colored chicken meat will show the darkened color of the marinade more than the beef.