1) 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.
2) 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.
3) 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 250ml 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 1.5 to 2 litres of broth). Stir in the peas. Mix in the Parmesan. Season with salt and pepper, to taste.