Finely chop the spinach. In a mixing bowl combine spinach, cheese, pepperoni, basil and pepper; set aside. Clean mushrooms; remove stems. Place open side up on a lightly greased cookie sheet; brush with margarine. Spoon 2 tbsp spinach mixture into each.
Bake in a 350 oven about 12 minutes or until heated through (Or place mushrooms on the unheated rack of a broiler pan. Broil 4 inches from heat for 3 to 4 minutes). If desired, garnish with fresh basil.