How to Make It
In a large sauce pan, melt the butter. Add the leeks and cook over moderate heat, stirring occasionally until softened, about 3 minutes. Add the rice and cook, stirring, for 3 minutes. Add the stock, salt, and pepper and bring to a boil. Cover and simmer over low heat until the broth is absorbed and the rice is tender, about 45 minutes. Remove from the heat and let stand, covered, for 10 minutes. Fluff with a fork.
In a skillet, heat the oil over moderately high heat. Add mushrooms and sauté until they are browned and any residual liquid has evaporated, about 5 minutes. Season with salt and pepper. Stir the mushrooms into the rice and serve.
If eating a gluten-free diet, be sure to use gluten-free stock.