Preheat oven to 350° F. Place 2 sage leaves in the center of each piece of prosciutto, then wrap each cutlet in the prosciutto, as shown. Put the flour on a plate and season with the pepper. Dredge the prepared cutlets in the flour. Heat the oil and 1 tablespoon of the butter in a large skillet. Once the butter is foaming, add 4 of the cutlets, seam-side down. Cook over medium-high heat until golden brown, about 3 minutes per side. Transfer to a baking dish. Add another tablespoon of the butter to the pan and brown the remaining cutlets. Transfer to the baking dish, then cook in oven for 8 minutes. Reduce pan heat to medium-low and add the Marsala. Stir constantly, scraping up the browned bits. Simmer for 3 minutes. Add the capers and parsley. Stir in the remaining butter. Spoon the sauce over the chicken and serve with sautéed spinach.