4 Portobello mushroom caps
2 Tbsp balsamic vinegar
1 Tbsp low sodium soy sauce
1 Tbsp olive oil
1 Tbsp chopped rosemary
1 ½ tsp steak seasoning
4 slices red onion
4 oz reduced fat Swiss cheese, thinly sliced
4 slices tomato
½ avocado, thinly sliced
4 leaves baby spinach or arugula
4 whole wheat buns
To prepare the mushrooms, remove and discard the stems and scrape out the gills. Wipe caps clean with a damp paper towel.
In a large bowl, whisk together vinegar, soy sauce, oil, rosemary, and 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.
When the grill is hot, brush the grate with oil. 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.
Top 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.
To finish, place the spinach and grilled Portobello mushrooms on the buns and top with the grilled onions, sliced tomato and avocado.