Preheat your oven on 250°C. (Use the grill-function, if it has one). Cut onion and tomato in thin slices. Heat the butter in a small pan. When it starts to bubble, it's time to put the portobello mushroom in. Fry it for around 5 minutes on each side.
Add the cheese on top of the portobello and put the pan in the oven until the cheese is melted and turned golden brown. While the mushroom is baking, heat the olive oil in another pan, then add the prawns and fry them until they turned red.Squeeze half a lime over the prawns and add some salt. Mix mayonaise with the juice of the other half of the lime, chiliflakes and black pepper. Roast the burgerbun-halfs.
Spread the chili-lime mayonaise on the insides of the bread, then stack all ingredients: first lettuce, then mushroom, now prawns, tomato, onions and the tophalf of the bun.