Heat the oven to 375°F. Line a rimmed baking sheet with parchment paper. In a large bowl, combine the ground turkey, breadcrumbs, egg, onion, 1 tablespoon milk, salt, pepper, and thyme. Mix gently just until combined.

Shape the mixture into a compact loaf on the prepared baking sheet. Bake for 25 minutes while you prepare the glaze and potatoes.

In a small saucepan, stir together the cranberry sauce, balsamic vinegar, and brown sugar. Bring to a gentle simmer over medium-low heat, stirring occasionally, then cook for 3 to 5 minutes until glossy and slightly thickened.

Spoon about half of the cranberry balsamic glaze over the meatloaf. Return it to the oven and bake for 15 to 20 minutes more, until the center reaches 165°F. Let it rest for 5 minutes before slicing.

While the meatloaf bakes, place the potatoes and garlic cloves in a pot and cover with cold water. Bring to a boil, then reduce the heat and simmer until the potatoes are very tender, about 15 to 20 minutes. Drain well.

Return the potatoes and garlic to the pot. Add the butter and warmed milk, then mash until smooth. Serve the meatloaf with the mashed potatoes and the remaining cranberry balsamic glaze.