1. Wash and slice potatoes. Finely chop onion and garlic.
2. Divide thinly sliced vegetables evenly and use to fill mushroom caps, with the herb butter.
3. Chargrill steaks to desired degree of doneness while sautéing potato, onion and garlic. Allow meat to rest.
4. Place filled mushroom caps in 360ºF oven for 8 minutes or until cooked. Place potatoes and steak on serving plates with mushroom caps.
5. Garnish with sautéed onion rings and fresh rosemary.