You'll want to very gently scrape out the mushroom gills and reserve them for the mushroom stuffing. Gently cut the stem, then start to scrape out the gills with a spoon. Try to be as gentle as possible because you can crack the mushroom cap and you don't want that to happen or the stuffing will spill out. Leave about 1/4 to 1/2 an inch of portobello cap so that you have ample room to stuff the mushroom but the mushroom isn't so thin that it will collapse. Once you have your mushrooms scooped out, you're good to start work on the stuffing.