Heat the oven to 375°F. Line a rimmed baking sheet with parchment paper.
Warm the olive oil in a small skillet over medium heat. Add the diced onion and cook for 4 to 5 minutes, stirring occasionally, until softened. Set aside to cool slightly.
In a large mixing bowl, combine the ground turkey, almond flour, egg, Worcestershire sauce, garlic powder, salt, pepper, 2 tablespoons of the ketchup, and the cooled onion. Mix gently until evenly combined.
Shape the mixture into a compact loaf on the prepared baking sheet. Avoid packing it too firmly, so the meatloaf stays tender.
In a small bowl, stir together the remaining ketchup, apple cider vinegar, and brown sugar. Spread the glaze over the top and sides of the loaf.
Bake until the center reaches 165°F on an instant-read thermometer, about 45 to 55 minutes. Let the meatloaf rest for 10 minutes before slicing.