To make the soup, heat the oil in a large pan, add the onion, leek and potato and cook for 4-5 minutes or until softened and lightly coloured.
Add the stock and seasoning, bring to the boil then cover, reduce the heat to simmer for 15 minutes or until the potato is tender.
Whiz with a hand blender to a smooth consistency then reheat, stir in the cream or crème fraiche and adjust the seasoning, to taste.
Meanwhile for the Welsh Rarebit, crumble the cheese and mix with the butter, Worcestershire sauce, mustard and ale or milk. Spread evenly over the toast and cook under a preheated grill for 1-2 minutes to melt the cheese and tinge brown in places.
Ladle the hot soup into bowls and garnish with chopped parsley. Cut the Welsh Rarebit into triangles to enjoy with the soup.