Slice mushrooms and red onion thinly.

Heat olive oil in a pan, sauté onion and mushrooms until soft.

Lightly toast bread slices.

Place blue cheese and a lump of butter on each toast.

Air fry at 180°C for 5 minutes until cheese is melted.

Drizzle balsamic vinegar over the hot toast.

Pile mushroom and onion mixture on top with salt and pepper and serve.