Clean and chop the 1/2 lb mushrooms and place in a bowl; add the garlic, marjoram, parsley, cheese, breadcrumbs, salt, and pepper, and mix well.
Add the egg and blend thoroughly until of a thick, homogeneous consistency.
Separate the stems from the caps of the 4 mushrooms; hollow out just enough of the cap to form somewhat of a bowl in which to place the filling. Chop what you have removed from the hollow and add to the filling, blending thoroughly.
Fill the mushroom caps with the filling, spreading it carefully so that it creates an even layer across the cap.
Sprinkle the caps with oil, and place in a 400 degree oven for 20 minutes.
Serve hot.