In a medium mixing bowl, stir together the spinach, cream cheese, onion, parmesan cheese, and black pepper. Set filling aside.
On a lightly floured surface, unroll pizza dough into a 15-inch square. With a knife, cut dough into twenty-five 3-inch squares. Fill each dough square with 1 rounded teaspoon of filling. Stretch dough over filling to form a triangle. Press edges of dough well, with a fork to seal.
Line a baking sheet with foil, grease lightly. Place calzones on baking sheet, and prick tops with a fork. Brush milk over the calzones.
Bake in a 425-degree oven, until golden (8-10 minutes). Let stand for 5 minutes before serving. Serve with spaghetti sauce if desired.