5. Remove portobello’s stalks, and place them down-side into a baking tray. Bake them for 3-5 minutes, to slightly often at 375F. Be careful, when they begin to release juice, take them out of the oven, the time will vary due to the size of each portobello. At this point, you are looking to soften the mushrooms. Once tender, remove them from the oven.