Calling all cheese lovers! This cheesy mushroom toastie is absolutely mouth-watering.
Heat the oven to 435 °F. Chop and fry the mushrooms in butter until soft and nicely browned. Fry the onion in butter and sugar until caramelized. Spread butter on the bread and add the cheese slices. Add the fried mushrooms and caramelized onion. Fry the sandwich in a cast-iron skillet with plenty of butter until golden. Top the sandwich with chopped parsley and serve with truffle mayonnaise.