Preheat the oven to 350F. In a skillet, cook and brown the sausage for 10 minutes over medium-high heat until browned and cooked through. Add the onion and celery and cook for 3 more minutes until slightly soft.
In the bottom of a large casserole dish, combine the stock, soup, rice, and sausage mixture. Add the crushed red pepper flakes if using. Stir together, and smooth out the top.
Cover the casserole tightly with foil and bake for 1 hour, then taste the rice for doneness. If the rice is cooked through, remove from the oven and let sit, covered, for 15 minutes until serving. Add a bit more cooking time if needed, or a splash of extra stock if the rice is uncooked and the casserole is very dry.
Serve in shallow bowls, and enjoy!