In a large bowl, mix together the ground turkey, egg yolk, bread crumbs, parmesan, salt and pepper and crushed red pepper flakes until evenly combined. Form the mixture in 8-10 evenly sized meatballs.
Drizzle the olive oil into a large non-stick skillet over moderate heat. Once hot, brown the meatballs in batches. Allow the meatballs to brown on one side before turning, about 5 minutes. Once browned, remove the meatballs from the skillet to a clean plate.
Add butter to the skillet and a little more oil if the skillet appears dry. Add the onion and celery. Cook until just beginning to softened. Stir in the garlic and mushrooms and cook until the mushrooms have begun to soften, about 5 minutes. Stir in the rice. Cook long enough to toast the rice slightly.
Add the chicken stock and then return the meatballs to the pot. Stir and bring to a simmer. Cover, lower the heat and cook until the rice has absorbed all the liquid and has softened, about 20 to 25 minutes. Uncover and stir in the fresh herbs and salt & pepper to taste. Serve immediately and enjoy!