BBC Food - Recipes - Mushroom and mozzarella burger with relish

Ingredients

• 1 tbsp olive oil
• 1 onion , finely diced
• salt and pepper
• 1 garlic clove, crushed
• 200g/7oz tinned tomatoes
• 1 tbsp white wine vinegar
• 25g/1oz butter
• 2 onions , finely sliced
• 4 large flat mushrooms , stalk trimmed so it doesn’t come above the edge of the mushroom
• 1 ball mozzarella
• 4 burger buns, sliced in half