Soak the dried porcini in hot tap water for about an hour.
In a pot, prepare (or heat) a light vegetable broth and keep it hot, over the minimum heat.
Coarsely chop the parsley and grate the parmesan, and set them aside.
Peel and thinly chop the onion.
Heat the olive oil in a medium skillet, add the onion and brown it for a couple of minutes.
Add the rice and toast it a couple of minutes, add the porcini and cook them one minute more.
Add a ladleful of broth and cook the rice over medium/low heat until it has absorbed almost all the liquids. Keep adding one ladleful of broth at the time, until the rice is cooked (al dente). Don’t add broth before the previous ladleful has been almost totally absorbed. If you finish broth, you can continue with hot water.
Once the rice is cooked, turn off the heat, add salt and pepper to taste, and pour in the butter and the grated parmesan. Mix with a wooden spoon until well amalgamated.
Sprinkle the porcini risotto with parsley and serve it immediately.