In a large bowl, whisk together vinegar, soy sauce, oil, rosemary, and Karns Butcher's Love Steak Seasoning.

Place the mushroom caps in the bowl and toss to evenly coat. Let stand at room temperature for 20 to 30 minutes, turning a few times.

Heat the grill over medium heat. When hot, brush the grate with oil. This will keep the mushrooms from sticking.

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