Clean mushrooms with a damp paper towel.
Remove and finely chop stems.
Set mushroom caps and chopped stem aside.

Drain spinach well and press between
paper towels to remove excess moisture.

Dip mushroom caps in melted butter;
place in a 13 x 9 x 2 inch baking dish, cap side down.
Saute reserved chopped mushroom stems and onion
in any remaining melted butter in a skillet.
Add spinach; stir well. Set aside.

Combine egg yolk, 2 1/2 tablespoons Parmesan cheese,
salt, nutmeg and pepper in a small bowl;
stir well. Add to spinach mixture, stirring well.

Spoon spinach mixture evenly into mushroom caps;
sprinkle evenly with remaining 1 1/2 tablespoons Parmesan cheese.
Bake at 325 degrees for 20 to 25 minutes or
until stuffed mushrooms are thoroughly heated