Preheat the oven to 400℉. Prep a baking dish with cooking spray.
In a medium bowl, add ground turkey, Panko bread crumbs, egg yolk, onion, salt, and pepper. Combine ingredients using a fork or your hands.
Using a Tbsp, scoop mixture out the bowl and form into balls. You will end up with 8-10 meatballs. Place in the baking dish.
Bake for about 20 minutes until meatballs are baked through.
Heat a skillet over medium heat and melt the butter. Once the butter is melted, add the garlic. Sauté the garlic for one minute. Add the flour to the pan and stir until the flour is well mixed with the butter. Once the flour and butter begin to bubble, add the stock and continue stirring. Season with salt and pepper. Continue to cook over low heat. As the flour cooks, the sauce will thicken.
Add sour cream to the sauce and stir until well combined. Continue to heat and allow the sauce to simmer.
Once the meatballs are cooked through, pull from the baking dish and add to the skillet along with sauce. Stir and coat the meatballs with the sauce.
Garnish with parsley. Serve warm with noodles or over mashed potatoes.