1. In a small pot, add quinoa to 1 cup boiling water. Cover, reduce heat and cook 15 minutes. Let stand, covered, 5 minutes.
2. Heat oven to 400 degrees . In a sided skillet, cover potatoes with 1 inch cold water. Bring to a boil. Reduce to a simmer, cover and cook 10 minutes, until fork-tender. Drain potatoes and return to pot. Mash and stir in milk, sour cream, 1 tbsp of the butter, 1/4 tsp of the salt and 1/8 tsp of the pepper. Cover and set aside.
3. Finely chop 1 cup of the mushrooms. Combine with cooked quinoa, turkey, egg, Parmesan, parsley, 1/4 tsp plus 1/8 tsp of the salt and 1/4 tsp of the pepper. Form into twenty 2-inch meatballs and place on a rimmed sheet coated with nonstick cooking spray. Bake at 400 degrees for 15 minutes, until cooked through.
4. Meanwhile, slice remaining mushrooms. Add to remaining 1 tbsp butter in a saucepan over medium heat; cook 8 minutes, stirring several times. Stir in flour; cook 1 minute. Whisk in stock and bring to a boil. Reduce to a simmer; cook 3 minutes, until thickened. Season with remaining 1/8 tsp salt and 1/8 tsp pepper.
5. Serve meatballs and gravy over mashed potatoes.