portobello mushroom caps: 4， balsamic vinegar: 2 tbsp， low sodium soy sauce: 1 tbsp， olive oil: 1 tablespoons， chopped rosemary: 1 tbsp， steak seasoning like Montreal Steak Grill Mates: 1-1/2 tsp， red onion: 4 thick slices， reduced fat Swiss, sliced thin (Alpine lace): 4 oz， tomato: 4 thin slices， avocado, sliced thin: 1/2， baby spinach: ， whole wheat low calorie buns (I used Martin): 4
1In a large bowl, whisk together vinegar, soy sauce, oil, rosemary, and Montreal steak seasoning. Place the mushroom caps in the bowl and toss with sauce, using a spoon to evenly coat. Let stand at room temperature for 20 to 30 minutes, turning a few times.
2Heat the grill or indoor grill pan over medium heat. When hot, brush the grate with oil or lightly spray the grill pan. Place the mushrooms on the grill, reserving marinade for basting. Grill for 5 to 7 minutes on each side, or until tender, brushing with marinade frequently.
3Top the mushrooms with cheese during the last minute of cooking. While the mushrooms cook, grill the onions about 1 minute on each side and grill the buns until toasted.
4To finish, place the spinach and grilled portobello mushrooms on the buns and top with the grilled onions, sliced tomato and avocado.