1 coarsely chopped white onion
1 oz finely chopped fresh rosemary
4 oz low sodium soy sauce
6 cloves garlic
2 oz virgin olive oil
2 oz balsamic vinegar
1 oz Dijon mustard
1 teaspoon black pepper
2 lb sirloin steak
8 oz sliced mushrooms
Fully mix onion, rosemary, soy sauce, garlic, balsamic vinegar, Dijon mustard and black pepper in low bowl. Reserve a quarter of the marinade for cooking the mushrooms. Put the steak in the remaining marinade and refrigerate for eight hours to overnight, turning over the steaks a couple times during this time.
Remove meat from the marinade and barbeque steaks to one degree less of doneness than desired, remembering the meat will continue to cook after it's off the grill. Heat the extra marinade to a simmer and add the mushrooms to saute them. Top the steaks with the mushrooms and remaining marinade, and serve.