Heat the oven to 350°F. Combine the chicken, ½ of the onion, rice, garlic powder, Worcestershire, and egg. Season with salt and pepper. Set aside.
Heat a large skillet over medium high heat and coat with nonstick cooking spray. Add the remaining ½ onion and cook until golden brown, stirring occasionally, 5 to 7 minutes. Add the mushrooms and cook, stirring often, until tender, about 5 minutes. Add the flour and stir to coat the vegetables. Add the stock and stir to combine. Season with salt and pepper.
Reduce the heat to simmer. Scoop the reserved chicken mixture into the simmering sauce. Roll to coat or spoon over some of the sauce. Transfer to the oven. Cook, spooning the sauce over once or twice during baking, until the rice is cooked through and the sauce has thickened, 30 to 45 minutes. Remove from the oven and roll the meatballs in sauce to coat. Serve immediately.