Melt the butter in a pot over medium heat. Add the parsnips and onion and cook until softened, about 4 to 5 minutes. Mix in the flour until well combined. While stirring, slowly pour in the stock. Mix in the cubed pear and rosemary. Bring to a gentle simmer; gently simmer the soup until the parsnips are very tender, about 10 minutes. Purée the soup in a food processor or with a hand-held immersion blender. Return the soup to a simmer; season with salt and pepper. Ladle the soup into heated bowls. Place ¼ cup of cheese in the centre of each bowl. If desired, garnish each bowl with a slice or two of fresh pear and a rosemary sprig.