1 garlic clove, crushed
4 large flat mushrooms, stalk trimmed so it doesn鈥檛 come above the edge of the mushroom
1 onion, finely diced
2 onions, finely sliced
200g/7oz tinned tomatoes
salt and pepper
1 tbsp olive oil
1 tbsp white wine vinegar
25g/1oz butter
1 ball mozzarella
4 burger buns, sliced in half