Adapted from a Mary Ann Esposito recipe. The original called for Italian (not Danish) fontina cheese, but I had some wonderful sheep's milk feta to use up, so that's what is used here. Sheep's milk feta from Bulgaria is soft, rich, and creamy. I have two small individual baking dishes which each hold a Portobello very nicely, so use whatever works for you. I cover my little baking dishes with foil. Serve quarters as appetizers, whole mushrooms as a vegetarian main dish.