Preheat the grill to high. Put the haddock on a baking sheet and grill for 18 mins until opaque and cooked through. Cover, set aside and keep warm. Put the spinach in a colander and pour over boiling water to wilt. Use the back of a spoon to press out excess liquid, then set aside.
In a nonstick saucepan, mix the mustard, flour and 2 tsp milk into a paste. Pour in the rest of the milk and the Worcestershire sauce and cook over a low heat, stirring, for 1 min. Tip in the cheese and stir until melted, taking care not to let the mixture boil. Remove from the heat and set aside to cool a little.
Lightly toast the bread on both sides and place on a baking sheet. Divide the spinach between the toasts and top each with a piece of haddock. Beat the egg yolk into the warm cheese mixture and spoon over the haddock. Grill for 2-3 mins until golden and bubbling. Divide the watercress salad and orange slices between 4 plates and serve with the haddock rarebit, with a little oil drizzled over the salad to serve.
Tip: Swap the milk for beer or cider to give your rarebit a richer taste.