Portobello Mushroom Burger

Ingredients

• 2 Hamburger buns (split)
• 2 tbsp balsamic vinaigrette
• ½ tsp black pepper
• Salt (as required)
• 2 large mushroom caps
• 2 tbsp olive oil (+more)
• 1 onion (sliced)
• Romaine lettuce
• 2 garlic cloves (finely chopped)
• Swiss cheese (sliced)
• 1 large ripe tomato (sliced)