Preheat oven to 200°C/180°C fan/gas mark 6. Put onion, half of the garlic, 2 teaspoons of oil, the balsamic vinegar and black pepper in a small, non-stick roasting tin; toss to mix well.

Spread out in a single layer; nestle mushrooms, stem-side up, in amongst onion mixture. Combine remaining garlic and oil; brush tops of mushrooms with garlic oil mixture. Bake in oven for 15–20 minutes or until vegetables are softened and onions are just starting to caramelize.

Remove from oven; combine cheese, parsley and chives. Sprinkle herby cheese on top of mushrooms. Return to oven; bake for about 5 minutes or until cheese is melted.

Fill each bread roll with some roasted onion mixture, topped with two cheesy mushrooms (remember to drizzle over any delicious cooking juices, too). Add a tomato slice to each roll, if you like. Serve with remaining tomato slices and a mixed green leaf salad alongside (if using).