1. Cut beef steak into 1 <-inch pieces. Combine water, red wine vinegar, Dijon mustard, garlic cloves, 2 teaspoons vegetable oil and black pepper and marinate for 6 hours or overnight.
2. Soak bamboo skewers in water for 10 minutes, drain. Remove beef from marinade and discard marinade.
3. Alternately thread beef, onions, squash, and bell peppers evenly onto skewers. Place kabobs on grill over medium coals. Grill, uncovered 8 to 10 minutes for medium-rare to medium doneness, turning occasionally.
4. Heat 2 teaspoons oil in skillet until hot. Add mushrooms; cook and stir until tender. Remove and keep warm. Meanwhile, cook rice according to package directions. When done, stir in mushrooms.
5. Serve kabobs over mushroom wild rice.