Step 1

In a large bowl, whisk together vinegar, soy sauce, oil, rosemary, and Montreal steak seasoning.

Step 2

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.

Step 3

Heat the grill or indoor grill pan over medium heat. When hot, brush the grate with oil or lightly spray the grill pan.

Step 4

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.

Step 5

Top the mushrooms with cheese during the last minute of cooking.

Step 6

While the mushrooms cook, grill the onions about 1 minute on each side and grill the buns until toasted.

Step 7

To finish, place the spinach and grilled portobello mushrooms on the buns and top with the grilled onions, sliced tomato and avocado.