Heat the oven to 400°F. Line a rimmed baking sheet with parchment paper or lightly grease it. Lightly grease 8 cups of a standard muffin pan.
In a large bowl, combine the ground turkey, grated apple, diced onion, egg, breadcrumbs, 2 tablespoons of the BBQ sauce, Dijon mustard, garlic powder, salt, and black pepper. Mix gently until just combined.
Divide the turkey mixture among the 8 prepared muffin cups, pressing it in lightly. Brush the tops with 2 tablespoons of the remaining BBQ sauce.
Toss the Brussels sprouts with the olive oil and spread them on the prepared baking sheet, cut sides down where possible. Place the muffin pan and baking sheet in the oven. Roast for 20 to 25 minutes, until the sprouts are browned and tender and the meatloaf muffins reach 165°F in the center.
Brush the muffins with the remaining BBQ sauce during the last 5 minutes of baking. Let them rest for 5 minutes, then serve with the roasted Brussels sprouts.