Preheat oven to 220°C. Thinly slice mushrooms and onion. Toss on a lined oven tray with balsamic vinegar and olive oil. Season and roast for 20-25 minutes until soft and caramelised.

Brush the outside of sourdough with olive oil or lightly spread with butter. Once mushrooms and onion have roasted, fill the plain side of two slices, top with slices of cheese, then place another slice of sourdough on top.

Heat a dry fry pan on medium-low heat. Cook sandwich for 3-4 minutes each side, until bread is golden, and cheese has melted.

Serve, with your favourite relish or pickle.