Thaw spinach and press out liquid; finely chop. In a mixing bowl combine spinach, cheese, pepperoni, basil, and pepper. Clean mushrooms; remove stems. Place open side up on a lightly greased or parchment-lined baking sheet; brush with margarine. Spoon 2 tablespoons spinach mixture into each.
Bake in a 350 degree F oven about 12 minutes or until heated through. (Or, place on the unheated rack of a broiler pan. Broil 4 inches from the heat for 3 to 4 minutes.) If desired, garnish with fresh basil. Makes 12 appetizers.