In 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.

Heat 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.

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.