Preheat oven to 400°. In a shallow bowl, mix bread crumbs and Parmesan cheese. , Sprinkle tops of cutlets with salt and pepper. Layer end of each with Gouda cheese and spinach. Fold cutlets in half, enclosing filling; secure with toothpicks. Brush mustard over outsides of pork; dip in bread crumb mixture, patting to help coating adhere. , Place on a greased foil-lined baking sheet. Bake until breading is golden brown and pork is tender, 12-15 minutes. Discard toothpicks before serving.