Preheat oven to 375F.
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.
While the tomatoes and onion are roasting away, get the roux started: Melt the butter in a medium-size skillet over medium-high heat. Sprinkle the flour over the melted butter, cook for 1-2 minutes, then add the balsamic and stock to the pan, whisking to incorporate. Cook until thickened, then remove it from the heat and reserve.
Remove the mini meatloaves and the tomatoes from the oven and set the meat aside to rest for about 5 minutes.
Add the roasted tomatoes and onion to the sauce and break up the veggies with a potato masher. Serve one mini meatloaf per person with a hearty ladle of the tomato balsamic and some Mac & Seafood Au Gratin alongside.