Put your pan on medium-high heat until quite hot.
Meanwhile, slice the mushrooms (and chop the garlic finely).
Put a knob of butter in the pan, add the mushrooms (and garlic), and season with salt and pepper. Cook for a couple of minutes, stirring or turning the mushrooms after the first minute.
Toast the sourdough and separate the egg yolk while the mushrooms are sautéeing.
Serve the hot mushrooms on the lightly toasted sourdough, topped with the raw egg yolk.
Crack those yolks open and enjoy!