In a food processor, combine one small finely chopped onion, parsley, and a soft bun, pulsing until crumbly. Transfer the mixture to a bowl and add 1.1 pounds of ground turkey, along with ½ teaspoon each of salt and pepper. Pulse gently to combine, ensuring not to overblend. Form the mixture into 12-14 equally-sized meatballs, about 1-1.5 inches in diameter, and set aside.

Heat 2 teaspoons of oil in a large non-stick pan over medium heat. Once hot, carefully add the meatballs to the pan, browning them for about 4 minutes while gently turning frequently for an even golden color. The meatballs should be nicely browned but not fully cooked through. Once browned, remove the meatballs from the pan and set them aside on a plate.

In the same pan, add a little more oil if needed and heat over medium heat. Finely chop 12 ounces of regular white mushrooms using the food processor and set aside. Add one finely chopped medium onion to the pan, cooking for about 3 minutes until softened. Then, stir in the chopped mushrooms, cooking until their juices are mostly absorbed, about 5-7 minutes.

Sprinkle 2 tablespoons of all-purpose flour over the mushroom and onion mixture, stirring to coat for about 1 minute; this will create a flavorful base. Gradually add 1¼ cups of stock, followed by ¼ cup of light cream. Stir continuously, bringing the mixture to a gentle simmer until the sauce thickens, about 3-5 minutes.

Once the sauce has thickened, remove the pan from the heat and stir in 2 tablespoons of sour cream for added creaminess and tang. Taste and adjust the seasoning with additional salt and pepper as needed. The sauce should be rich and velvety.

Carefully place the browned meatballs back into the sauce, ensuring they are evenly coated. Cover the pan and lower the heat to a simmer, cooking for 10-12 minutes until the meatballs are cooked through.