In a small bowl, combine the first 8 ingredients (oil through pepper). Place beef cubes and mushrooms in a large, zipper topped plastic bag. Add marinade and seal bag. Marinate in refrigerator for 1 to 5 hours.
Preheat grill to medium-high heat.
Thread beef onto skewers, alternating with mushrooms and onions. Sprinkle with sea salt to taste.
Grill for 3 minutes on each side or until meat thermometer registers 125 - 130°. Transfer skewers to platter and let sit for 5 minutes before serving.