Heat the oven to 400°F. Line a rimmed baking sheet with parchment paper. In a large bowl, combine the ground turkey, grated apple, egg, almond flour, sage, garlic powder, ½ teaspoon of the salt, and the black pepper. Mix gently until just combined.
Shape the mixture into a compact loaf in the center of the prepared baking sheet. Avoid packing it too tightly so the meatloaf stays tender.
Toss the halved Brussels sprouts with the olive oil and remaining ¼ teaspoon of salt. Arrange them cut-side down around the meatloaf, leaving a little space between the pieces.
Bake until the meatloaf reaches 165°F in the center and the Brussels sprouts are browned and tender, about 35 to 45 minutes. If the sprouts finish before the meatloaf, transfer them to a serving dish and let the meatloaf continue baking.
Let the meatloaf rest for 5 to 10 minutes before slicing. Serve with the roasted Brussels sprouts.