Heat the oven to 400°F. Line a rimmed baking sheet with parchment paper or lightly oil it.
Heat 1 teaspoon of the olive oil in a skillet over medium heat. Add the chopped mushrooms and onion. Cook for 7 to 9 minutes, stirring occasionally, until softened and most of the liquid from the mushrooms has evaporated. Set aside to cool slightly.
In a large bowl, combine the ground turkey, cooked mushroom mixture, egg, breadcrumbs, milk, parsley, thyme, salt, and black pepper. Mix gently until just combined. Shape the mixture into a compact loaf and place it on one side of the prepared baking sheet.
In a bowl, toss the halved potatoes with the remaining olive oil and garlic powder. Arrange them cut-side down on the other side of the baking sheet, leaving space around the meatloaf.
Bake for 40 to 50 minutes, turning the potatoes once, until they are browned and tender and the center of the meatloaf reaches 165°F. Rest the meatloaf for 5 to 10 minutes before slicing. Serve with the roasted potatoes.