How to Make It
In a large, heavy saucepan, heat the olive oil until shimmering. Add the onion, season with salt and cook over high heat, stirring frequently, until softened, 4 to 5 minutes. Add the rice and cook, stirring, for 1 minute. Add the rosemary and 2 cups of the stock and bring to a boil. Cover, reduce the heat to moderately low and simmer for 5 minutes. Stir in another 2 cups of the stock and bring to a boil over high heat. Cover, reduce the heat to moderately low and simmer for 10 minutes.
Season the shrimp with salt and pepper. Stir into the rice along with the mushrooms and the remaining 2 cups of stock and cook over moderate heat, stirring occasionally, until the shrimp are just cooked and the rice is tender, about 10 minutes. Discard the rosemary. Stir in the butter and cheese and season with salt and pepper. Sprinkle the thyme on top and serve immediately.