Bring the broth to a simmer in a heavy medium saucepan. Add the porcini mushrooms. Set aside until the mushrooms are tender, about 5 minutes. Keep the broth warm over very low heat.
Melt the butter in a heavy large saucepan over medium heat. Add olive oil. Add the onions and saute until tender, about 8 minutes. Add the white mushrooms and garlic. Using a slotted spoon, transfer the porcini mushrooms to a cutting board. Finely chop the mushrooms and add to the saucepan. Saute until the mushrooms are tender and the juices evaporate, about 5 minutes. Stir in the rice and let it toast for a few minutes. Add the wine; cook until the liquid is absorbed, stirring often, about 2 minutes. Add 1 cup of hot broth; simmer over medium-low heat until the liquid is absorbed, stirring often, about 3 minutes. Continue to cook until the rice is just tender and the mixture is creamy, adding more broth by cupfuls and stirring often, about 28 minutes (the rice will absorb 6 to 8 cups of broth). Stir in the peas. Mix in the Parmesan. Season with salt and pepper, to taste.
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