Warm the butter or olive oil in a large sauté pan over high heat. Add the fresh mushrooms, stirring occasionally, until they give off their liquid, about 6 minutes. Add the besciamella and the porcini and soaking liquid (if using). Season with salt, pepper, and nutmeg.
Spread the warm mixture on the bread slices and top with a dusting of Parmesan. Slip under a preheated broiler u