1. Heat grill to medium temperature. As grill is heating, in small bowl, mix soy sauce, olive oil, mustard, Worcestershire sauce, fresh herbs, and salt and pepper.
2. Place caps on grill and cook for five minutes, frequently basting with mixture. Turn caps over, baste often, and cook until caps are tender, about 5 minutes. You may place buns on grill and lightly toast if desired.
3. Serve one Portobello cap on each bun and serve with a variety of condiments, including lettuce, tomato, red onion, ketchup, mustard, mayonnaise, pickles, avocado, jalapenos, or pineapple rings.