Recipes: Grilled portobello mushroom burger

Preparation:   Preparation time: 30 mins Cooking time: 30 mins Serving: 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 steak seasoning like Montreal Steak Grill Mates 4 thick slices red onion 4...