Method

1. Preheat the oven to 180C/350F/Gas 4.

2. For the relish, heat a small frying pan over a medium heat and add the olive oil. Fry the onion and a pinch of salt in the oil for 4-5 minutes, or until the onion is soft. Add the garlic and cook for two minutes, then stir in the tomatoes and the vinegar. Gently simmer for five minutes, or until the mixture has thickened to a relish consistency. Taste and adjust the seasoning as necessary. Leave to cool.

3. For the onions, heat a frying pan over a medium heat and melt the butter in the pan. Add the sliced onion and a pinch of salt. Turn down the heat and leave the onions, stirring now and again, to cook for 25-30 minutes, or until dark golden in colour and completely collapsed.

4. For the rest of the burger, place the mushrooms with the stalk side facing up onto an oven tray, drizzle with the oil and season with salt and pepper. Bake in the oven for 10 minutes, or until beginning to soften. Meanwhile slice the mozzarella ball into four equal slices. After the mushrooms have cooked for 10 minutes, remove from the oven and place a slice of mozzarella into each of the mushrooms on top of the stalk. Put back into the oven for 5-7 minutes to melt the mozzarella.

5. To assemble, lightly toast the cut side of the burger buns. Spread with a little relish and then place the mushrooms on top of the bottom burger bun. Divide the onions among the burgers and add any remaining relish on top. Finish by sandwiching each burger with the remaining halves.