Remove the stems (and gills if desired) from the mushrooms and arrange, cap side up, on a rack over a rimmed baking sheet. Roast at 350F for about 20 minutes. When cool enough to handle, stuff as desired, then roast until filling is heated through and done to your liking. This process allows the moisture in the mushrooms to release and evaportate, plus it browns the mushrooms nicely for top flavor.