Bring the stock to a simmer in a heavy, medium saucepan. Remove from the heat, add the porcini mushrooms, cover, and let steep until the mushrooms are tender, about 5 minutes.
Using a slotted spoon, remove the mushrooms from the liquid and finely chop them. Cover the stock and keep warm over very low heat.
Melt the butter in a large, heavy saucepan over medium heat. Add the onions and cook until tender, about 8 minutes. Add the chopped porcini mushrooms, button mushrooms, and garlic, and saute until the mushrooms are tender and the juices evaporate, about 10 minutes.
Stir in the rice, making certain each grain of rice is coated with butter. Add the wine and cook, stirring often, until it's absorbed, about 2 minutes.
Add 1 cup hot stock and simmer over medium-low heat, stirring often, until the liquid has been absorbed, about 3 minutes. Add another cup stock and continue to cook, stirring often. Repeat, taking your time and stirring frequently, until you've used about 5 3/4 to 5 cups of the stock and the rice is barely tender and the mixture is creamy, about 28 minutes. If desired, add some or all of the remaining stock to achieve your preferred consistency.
Stir in the peas, if using. Mix in the Parmesan and season with salt and pepper to taste. Sprinkle with more Parmesan, if desired.