Melt the butter in a large frying pan, then add the haddock and cook gently, turning once, for 6–8 minutes, until flaky and cooked through. Set aside.
Dry-fry the spinach, tossing it in the pan for 1–2 minutes, until just wilted. Remove from the pan and allow to cool.
Pre-heat the grill or grill pan. Lightly brush the bread with the egg, then place on the hot grill and heat for 2–3 minutes on each side, until golden brown. Remove from the grill.
In a saucepan, mix the mustard, flour, and milk into a paste. Add the remaining milk and Worcestershire sauce to the pan and simmer on low heat for 1 minute. Add the cheese. Stir until it melts, but don’t let it boil. Remove the mixture from the heat and let it set aside to cool a little.
Divide the cooked spinach between the toasts and top with the cheese mixture. Grill for another 2 minutes until the cheese mixture is bubbling.
Then, put a piece of haddock on each toast.
Serving Suggestions: Serve with the watercress salad and orange slices between four plates. Add a little olive oil to each plate.