Preheat the oven to 350 degrees F (175 degrees C). Spray a loaf pan with cooking spray.

Spray a medium skillet with cooking spray. Add onion and garlic and saute until the onion is tender, about 5 minutes. Set aside to cool.

Mix ground turkey, cheese cubes, crushed crackers, 1/3 cup ketchup, egg, garlic salt, salt, and pepper in a large bowl. Add cooled onion and garlic mixture and mix until well combined. Transfer to the prepared loaf pan. Place the loaf pan on a baking sheet and cover with aluminum foil.

Cook in the preheated oven for 1 hour. Remove foil and drain any excess juices from the loaf pan. Top meatloaf with ketchup and cook, uncovered, until no longer pink in the center, about 30 more minutes. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C).