Heat the oven to 400°F. Line a rimmed baking sheet with parchment paper.
In a large bowl, combine the egg, breadcrumbs, milk, diced onion, thyme, salt, pepper, and 1 tablespoon of the balsamic vinegar. Add the ground turkey and mix gently until just combined. Avoid overmixing, which can make the loaf dense.
Shape the turkey mixture into a compact loaf in the center of the baking sheet. In a small bowl, stir together the remaining 1 tablespoon balsamic vinegar, maple syrup, and Dijon mustard. Brush half of the glaze over the loaf.
Toss the carrots, parsnips, and potatoes with the olive oil. Arrange them around the meatloaf in a single layer, keeping some space between the pieces so they roast rather than steam.
Roast for 25 minutes. Brush the meatloaf with the remaining glaze, then continue roasting until the vegetables are tender and the center of the meatloaf reaches 165°F, about 15 to 20 minutes more.
Let the meatloaf rest for 5 minutes before slicing. Serve with the roasted root vegetables.