Heat the oven to 350 F. Grease a rimmed baking sheet or roasting pan with the olive oil. Soak the bread crumbs in the milk in a large bowl until the milk is absorbed, about 5 minutes.
Add the meat, egg, Parmesan, parsley, garlic, onion, sage and some salt and pepper and gently fold everything into the meat with your hands or a rubber spatula. Mix just enough to distribute the flavorings without overworking the meat or it will become tough.
For meat loaf: Turn the meat mixture onto the prepared baking sheet, gently shaping it into an oval mound. Drape the bacon over the top if you're using it. Bake for 45 to 60 minutes, brushing occasionally with the rendered pan juices. When done, the meat loaf will be lightly browned and firm. To check, insert a quick-read thermometer into the center; it should read 160 F. Let the loaf rest for 5 minutes before slicing with a serrated knife.
For meatballs: Shape the mixture into balls of any size and put them on the prepared baking sheet or roasting pan. It's okay if they touch a little; they'll shrink as they cook. Figure that 2-inch meatballs will take 25 to 35 minutes to cook and add another 10 to 15 minutes for every additional inch in size.