Heat one tablespoon of the olive oil together with the butter in a frying pan and gently fry the mushrooms for 4-5 minutes, or until softened.
In a separate, non-reactive pan, heat another tablespoon of the oil and fry the tomatoes with the balsamic vinegar for 4-5 minutes, or until softened.
Bring a pan of water to the boil, then reduce the heat and add the vinegar. Using a wooden spoon, swirl the water to create a whirlpool, then crack the egg into the middle of the whirlpool. Poach for 2-3 minutes, or until the egg white is just set. Remove with a slotted spoon and drain on kitchen paper.
Heat the remaining olive oil in a pan and fry the prosciutto for 1-2 minutes, or until crisp.
To serve, place the toasted bread on a serving plate and spoon over the mushrooms, followed by the tomato and egg. Lay the crisp prosciutto over the top, then season with salt and freshly ground black pepper. Serve immediately.