Put the potatoes in a pot, cover with water, and bring to a boil over medium heat. Salt the water and boil the potatoes until tender, about 15 minutes. Drain the potatoes and put them back in the hot pot along with the spinach. Stir to combine, cover the pot, and let stand for 2 to 3 minutes to wilt the spinach. Add 1/2 cup of the milk, 2/3 cup of the cheese, and salt, pepper, and nutmeg to taste. Mash the potatoes and spinach and taste to adjust the seasoning.
In a small bowl, soak the bread in the remaining cup of milk. Put the meat in a bowl and add 1/3 cup cheese, the parsley, garlic, onion, marjoram or oregano, and salt and pepper, to taste. Squeeze the excess liquid from the bread and crumble it as you add it to the meat. Mix well to combine and form 4 oval loaves no more than 2 inches thick at the center. Heat the extra-virgin olive oil in a large skillet or Dutch oven over medium-high heat. Add the meat loaves to the skillet and cook for 3 to 4 minutes on each side until lightly browned. Pull the loaves out of the skillet and add the tomato paste. Stir for 1 minute, then stir in the stock and tomatoes. Return the loaves to the skillet, spoon the sauce over the top, and cover with a lid. Simmer over low heat until the loaves are cooked through, about 10 minutes more.
Set a loaf on each plate, spoon some spinach potatoes alongside, then drizzle the sauce over the top. Garnish with basil more grated cheese.