In a large bowl, combine the ground turkey, diced apple, egg, panko, Dijon mustard, sage, garlic, salt, and black pepper. Mix gently until evenly combined, taking care not to overwork the turkey.
Shape the mixture into a compact loaf about 6 inches long. Line the air fryer basket with a piece of perforated parchment, then place the loaf in the center.
Toss the Brussels sprouts with the olive oil. Arrange them around the meatloaf in a single layer, with the cut sides facing up where possible.
Air fry at 350°F for 20 minutes. Stir the Brussels sprouts and turn them so they brown evenly. Continue cooking for 8 to 12 minutes, until the meatloaf reaches 165°F in the center and the sprouts are tender with browned edges.
Carefully lift out the meatloaf and let it rest for 5 minutes before slicing. Serve with the Brussels sprouts.