Place dried porcini mushrooms in a bowl and cover with cold water. Allow to soak for at least 3 hours.

Finely slice the Portobello mushrooms and onion. Heat oil and butter in a pan on medium and cook mushroom and onion until softened, about 5 minutes. Slice garlic and add to the pan. Drain porcinis and reserve soaking liquid. Chop porcini and add to the pan, along with the strained liquid (just in case there is any grit). Continue to cook until most of the liquid has evaporated, season well, remove from heat and stir in chopped parsley and truffle oil.

Fry the eggs in a little olive oil, ensuring the centre is still a little runny. Slice buns in half and toast under the grill. To serve, place egg and mushrooms on the bun and add a little BBQ sauce or relish if you like.