They're gooey, stretchy and full of melty goodness!
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 dairy-free spread. 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.