Soak 3/4 cup of dried porcini mushrooms (3/4 ounce) in 3/4 cup of boiling water until softened, about 20 minutes. Rub the porcini to remove any grit and coarsely chop them. Reserve the soaking liquid. Add the porcini to the saucepan when the garlic is golden and cook for 1 minute. When the sauce is done, add the reserved porcini soaking liquid, stopping when you reach the grit, and cook until thickened, about 10 minutes.