Portobellini mushrooms are medium in size with caps averaging 5-7 centimeters in diameter and are connected to a thick stem. Slightly smaller than the portobello mushroom, but larger than a chestnut button mushrooms. Portobellini mushrooms contain fiber, niacin, copper, selenium, and phosphorus. Portobellini can be sliced and mixed into salads, soups, stews, pasta, rice dishes, grilled for fajitas, or baked into macaroni and cheese