Mushroom Risotto

1. Bring broth and water to a simmer in a 4-quart pot; keep broth at a bare simmer, covered.
2. Heat oil with 1 tablespoon butter in a 4-quart heavy saucepan over moderately high heat. Add mushrooms and sauté, stirring occasionally, until browned, about 4 minutes. Season with salt and pepper, then transfer to a bowl.
3. Cook onion in 2 tablespoons butter in saucepan over moderate heat, stirring, until softened, about 3 minutes. Add rice and cook, stirring, 1 minute. Add wine and cook, stirring, until wine is absorbed, about 1 minute.
4. Ladle in 1 cup simmering broth and cook at a strong simmer, stirring, until broth is absorbed, about 2 minutes. Continue simmering and adding broth, about 1/2 cup at a time, stirring frequently and letting each addition be absorbed before adding next, until rice is just tender and looks creamy, 18 to 20 minutes. (Save leftover broth for thinning.)
5. Remove from heat and stir in 1/2 cup cheese, remaining tablespoon butter, and salt and pepper to taste. Gently stir in mushrooms, then cover pan and let stand 1 minute. If desired, thin risotto with some of remaining broth. Serve immediately with remaining cheese on the side.