Put the dried porcini in a bowl, cover with water, and soak for at least 15 minutes.
Put the broth in a pot over high heat and bring to a boil. Lower the heat to maintain a very gentle simmer.
While the porcini are soaking, peel and finely chop the onion.
When the porcini are ready, put the onion and 2tablespoons of the butter in a heavy-bottomed braising pan and place over medium-high heat. Sauté, stirring occasionally, until the onion turns a rich golden color, about 5 minutes.
While the onion is sautéing, lift the porcini out of the water and squeeze the excess back into the bowl; do not discard the water. Rinse the mushrooms under running water, then coarsely chop them. Pour the soaking water into a small pot. Be aware that there may be some sand at the bottom of the bowl, so pour carefully or strain through a paper towel. Place over high heat and bring to a boil. Lower the heat to maintain a very gentle simmer.
Add the chopped porcini to the onion, season with salt, and stir a few times. Add the rice and stir until it is well coated. Add about 1cup of the hot broth and continue stirring. Add only enough broth to produce the consistency of a rather thick soup and wait until all the liquid is absorbed before adding more. As soon as the porcini water is hot, add that until you’ve used it all up, then go back to using the broth. Continue until the rice is al dente, 20 to 25 minutes.
Remove the risotto from the heat and stir in the remaining 1tablespoon butter and the Parmigiano. Serve at once.