Prepare the filling: Melt the butter in a large heavy sauté pan over medium heat. Add the onion and cook until translucent, about 7 to 10 minutes. Add the garlic if using for the last 2 minutes of cooking. Add the Swiss chard and cook until wilted, 2 to 3 minutes. Drain the chard mixture and squeeze out as much moisture as possible. Then finely chop the mixture and combine with the prosciutto and cheeses in a mixing bowl. Season to taste with salt and pepper, and nutmeg if using. Refrigerate until cold and firm.
Stuff each chop with
Heat the grill or broiler. Brush the chops lightly with oil and sprinkle lightly with salt and pepper. Grill or broil until the meat is medium-rare and the filling hot, 5 to 6 minutes each side.
Heat the tomato sauce to simmering. Spoon the sauce over the chops and serve with fried potatoes or baked polenta.