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1
Combine water and 1 cup stock in a small saucepan and heat until hot. Put mushrooms and 1 T oil in a bowl and pour hot liquid over them. Let soak for 30 minutes.
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2
Lift porcini out of soaking liquid, squeeze excess liquid back into bowl, and rinse well to remove any grit. Coarsely chop porcini. Pour soaking liquid through a fine sieve lined with a dampened paper towel into a large saucepan. Add remaining 4 1/2 cups stock and bring to a simmer. Reduce heat, cover, and keep at a bare simmer.
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3
Melt 2 T butter with remaining 1 T oil in a 4-quart saucepan over medium heat. Add onion and cook, stirring, until softened, about 3 minutes. Add rice and cook, stirring, for 2 minutes. Add 1/2 cup stock and cook at a strong simmer, stirring constantly, until stock is absorbed. Continue adding stock, about 1/2 cup at a time, stirring constantly and letting each addition be absorbed before adding next, until rice is tender and creamy-looking but still al dente, 18-20 minutes. There will be leftover stock.
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4
Stir in mushrooms, remaining 2 T butter, cheese, salt and pepper. If necessary, thin risotto with some of remaining stock. Serve immediately, sprinkled with extra cheese.
