Dice the onion. Also, remove the bottom of the stems from the mushrooms. Then, finely chop the mushrooms.
Add the olive oil to a skillet over medium heat on the stove. Then, add the finely chopped mushrooms and diced onion. Sauté them for about 5-7 minutes. After that, add the crushed garlic and sauté for an additional minute until fragrant.
Pour the balsamic vinegar into the pan to deglaze it. Scrape up any browned bits from the bottom of the skillet with a wooden spoon. Allow the vinegar to cook off for 1-2 minutes and reduce slightly. After that, remove the skillet from the heat and let it cool slightly.
In a large bowl, combine the ground turkey, sautéed ingredients, panko breadcrumbs, chicken broth, Italian herbs, salt, pepper, tomato paste, Worcestershire sauce, and an egg. Mix gently until combined, being careful not to over-mix.
On a parchment paper-lined baking sheet shape the mixture into a loaf, about 1½ to 2 inches high.
Bake the meatloaf at 350°F for 30-45 minutes, or until the internal temperature reaches 165°F. The cooking time will depend on the thickness of your meatloaf, ours was about 1½ inches high and took 30 minutes to cook.
Once done, let the meatloaf rest for about 10 minutes before slicing and serving.