The Best Grilled Portobello Mushroom Burger

Ingredients (12)

• 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 oz reduced fat Swiss
• 4 thin slices tomato
• 1/2 avocado
• baby spinach
• 4 whole wheat low calorie buns