In a shallow bowl, whisk together the soy sauce, balsamic vinegar, olive oil, rosemary, salt, pepper, powdered garlic, curry powder, and dry mustard. Place the mushroom caps in the marinade, and toss, using a spoon to coat them. Let sit at room temperature for 20-30 minutes, flipping the mushroom caps over periodically.
Heat a gas or charcoal grill to medium. When hot, brush the grate with oil. Arrange the mushrooms on the grill. Brush with marinade, and grill for 5-8 minutes on each side, until tender. Brush frequently with marinade. If using cheese, place the Swiss chesse on the portobello caps during the last minute of cooking. Place the red onions on the grill for 1-2 minutes, flipping once. Warm the buns on the grill if you like.
Serve the portabella mushrooms on the buns, topped with the grilled red onion, sliced tomato, and avocado. Enjoy!