1. Preheat oven to 350° F. Place the mushrooms on a baking sheet and drizzle with olive oil. Roast for about 10-12 min, or until they start to soften.

2. Meanwhile, mash the avocado and mix with tuna, lemon juice, and salt and pepper.

3. Once mushrooms are ready, split the tuna salad between them, top with tomatoes, and a slice of cheese each.

4. Place back in oven until the cheese is melted. Broil on low for a few minutes until the cheese just turns brown.