Preheat oven to 375ºF.
In a large bowl, mix ground beef, pork, veal, breadcrumbs, sage, parsley, scallions, Worcestershire sauce, egg, salt and a palmful of freshly ground black pepper.
Using a 6-cup muffin pan, divide the meat mixture into 4 equal parts and scoop the portions into four of the muffin cups. Bake the mini meatloaves until cooked through and golden on top, about 30 minutes.
While the mini meatloaves are cooking, toss the grape tomatoes and onion slices onto a baking sheet along with a drizzle of EVOO, salt and freshly ground black pepper. Once the meatloaves have been in the oven for about 15 minutes, put the tray of veggies in and roast until the onions are caramelized and the tomatoes have burst open, about 15 minutes.