- Place the vinegar, lemon juice, olive oil, herbs, pepper, and garlic in a medium bowl.
- Add Portobello mushrooms and marinate for 20 minutes.
- Preheat grill. Grill the mushrooms until nicely browned, about 6 minutes per side.
- Place the roasted red pepper and mayonnaise in a food processor or blender and mix until smooth.
- Toast buns on grill.
- Place one mushroom on the bottom of each bun. Place cheese on top, followed by lettuce and tomato. Spread the roasted red pepper mayonnaise on the top of the bun and serve.