Place the dried porcini in a bowl and pour the warm water over. Let stand until softened, about 10 minutes.
Meanwhile, heat the oil over medium heat in a 6 to 8-quart pressure cooker. Add the onions and cook until tender, about 5 minutes. Add the shiitake mushrooms and cook, stirring frequently, until crisp-tender, about 5 minutes.
Scoop the dried mushrooms out of the soaking liquid with your fingers, reserving the liquid. Rinse and coarsely chop the mushrooms. Strain the soaking liquid through a layer of paper towel. Add the chopped mushrooms to the pan along with the rice and stir to coat. Add the wine and cook until absorbed, about 30 seconds.
Add the reserved soaking liquid, the broth and salt and close the pressure cooker lid. Place the slow cooker over high heat until the pressure is up to high and the valve begins to rattle, and then reduce the heat to medium-low (pressure will still be up, but not as high). Cook for 8 minutes, and then remove from the heat. Allow the pressure to come down on its own and once it has, remove the lid.
Stir in the Parmesan and butter. Pass additional Parmesan and a peppermill at the table, if you like.
Recipe courtesy of The Fabulous Beekman Boys