1. Peel and finely chop the onion. Trim the bottom of the stem of the shiitake mushrooms, wash, and thinly slice. Brush away any dirt from the white mushrooms and thinly slice.
2. Trim the crust away from the bread and put the bread and milk in a small bowl. Mash the bread with your fingers until you have a smooth pulp. Put it in a mixing bowl with the ground beef, egg, nutmeg, and Parmigiano-Reggiano. Season with salt and pepper and mix everything together thoroughly with your hands.
3. Put the butter and vegetable oil in a heavy bottomed braising pan and place over medium high heat.
4. Spread the breadcrumbs on a plate. Shape the meat into a loaf 3/12 to 4 inches thick. Roll the loaf over the breadcrumbs until it is coated all around.
5. When the oil and butter are hot and the butter is just beginning to turn color, gently lower the meatloaf into the pan. Once it has browned on the bottom, carefully turn it over with two spatulas to brown the other side. Use the spatulas to lift the meatloaf out of the pan and set it aside on a plate. Be aware that since the loaf is not cooked through yet, it can come apart if you are not careful.
6. Turn the heat down to medium and add the onion. Sauté, stirring occasionally, until it turns a rich golden color, 3-5 minutes. Chop the sage and stir it in. Raise the heat to medium high, add the wine, and let it bubble away until it has completely evaporated. Add the mushrooms, season with salt and pepper, and cook, stirring occasionally, for 5 minutes.
7. While the mushrooms are cooking, peel the tomatoes (see page 000) and coarsely chop them. Add the tomatoes to the pan, season lightly with salt, and put the meat loaf back in. The easiest way to do this is to hold the plate over the pan and slide the meatloaf into the pan. Once the tomatoes have started bubbling, lower the heat so that they simmer. Cover the pan and cook for 45 minutes, turning the meatloaf every 15-20 minutes, until it is cooked through. Remove the cover and, if the sauce is still somewhat watery, raise the heat to medium high and cook, stirring often, until the sauce has reduced down enough to coat a spoon. Take the meatloaf out of the pan and cut into 1/2" slices. Place the slices on a serving platter and pour the sauce over them. Serve hot with some good crusty bread.
Note: This is a dish that will be still very good 2-3 days after it's made. Slice the meatloaf when you are ready to serve, then reheat it in its sauce over the stove, in a 350° oven, or in a microwave.