Heat the cooking oil in a pan, and add the mushrooms. Season the mushrooms to taste with salt. If the mushrooms are giving off a lot of water which can happen if you had to rinse them to remove grit, keep cooking until the water evaporates and the mushrooms start to brown.
Add the finely sliced or diced leek or shallot.
Add a knob of butter if the pan looks dry, cooking until the leek is tender and cooked, and the mushrooms are browning gently along the edges, and are fully cooked.
Finally, double check the seasoning, adjust as needed then add the parsley and spread the bread with the black trumpet butter, spoon the mushrooms on top and eat like the fungus eating glutton you are.