- With the rack in the middle position, preheat the oven to 425°F (220°C).
- Place the bread slices on a non-stick baking sheet. Toast in the oven for 5 minutes.
- Meanwhile, in a food processor, blend the cheese, crème fraîche, mustard, Worcestershire sauce and egg yolk until combined. Season with pepper (do not add salt as the cheese and mustard are salty enough).
- Cover the toasted bread with the onion confit. Using a knife or spatula, spread the cheese mixture over the bread and onion confit. Transfer to the oven and bake for 5 minutes or until the cheese mixture is melted and golden. Finish off under the broiler, as needed. Let cool for 5 minutes.
- Meanwhile, in a bowl, combine all of the ingredients. Add a few drops of Worcestershire sauce, to taste.
- Spread the topping over the toast. Cut into wedges and serve as a snack or alongside a salad for a light lunch.
Welsh rarebit, with its toasted bread smothered in melted cheese, is a take on a croque-monsieur or a croque-madame, as it is sometimes served topped with a fried egg.