Clean the mushrooms, remove the stems and cut into very thin slices. Chop the parsley and the garlic. Sauté the onions and the anchovy in a saucepan with 3 tbs. oil, add the mushrooms stems and the sweetbreads thinly sliced.
Salt, pepper and cook for about 10 mins, stirring frequently. When done, chop finely and place in a bowl. Remove the crust and add the white of the bread, softened in warm water, the cheese, egg, parsley and the garlic, and mix well.
Season with salt and pepper to taste. Add some breadcrumbs if the mixture is too soft.
Fill each mushroom cap with the prepared mixture, level off the tops with a spatula, place them in a baking dish, sprinkle with breadcrumbs and a small amount of oil, and cook in pre-heated oven for 20 mins. at 400ºF. Serve very hot.
