Prepare grill to approx. 350 degrees. Remove stems from mushrooms and with a spoon scrape out the gills. Rub tops of mushroom caps with olive oil. Place stem side up on a platter. Combine the onion, breadcrumbs, minced sage, pepper, salt and turkey in a mixing bowl. Divide mixture into four portions and pat into the mushroom caps. Place mushrooms on prepared grill and cook with grill top covered for 15 - 20 minutes or until cooked through. Top with cheese slices; grill until cheese is melted. Serve immediately and garnish with sage leaves.