Servings: 4 INGREDIENTS 4 portobello mushroom caps 2 tbsp balsamic vinegar 1 tbsp low sodium soy sauce 1 tablespoons olive oil 1 tbsp chopped rosemary 1-1/2 tsp vegetable stock 4 thick slices red onion Thinly sliced parmesan cheese 4 thin slices tomato 1/2 avocado, sliced thin baby spinach 4 buns   METHOD In a large