Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper.

In a skillet over medium heat, melt a tablespoon of butter and sauté the diced onion for about 5 minutes, adding thyme and garlic towards the end.

In a mixing bowl, combine ground turkey, breadcrumbs, eggs, parsley, kosher salt, pepper, Worcestershire sauce, and the cooled onion mixture.

Divide the mixture into eight equal portions and shape into small loaves, placing them on the baking sheet.

For the glaze, mix ketchup, balsamic vinegar, and brown sugar in a bowl and coat the tops of the meatloaves half of the glaze.

Bake for 15 minutes, brush with remaining glaze, and continue baking until the internal temperature reaches 150°F (65°C).

Remove from the oven and let rest for 5-10 minutes before serving.