Remove haddock fillets from packaging and thaw completely in the refrigerator if frozen
Preheat oven grill (broiler) to high
Place haddock fillets in a saucepan, add just enough water to cover, and gently poach over low heat until opaque and just cooked through (about 5–7 minutes)
Lift haddock fillets from pan and set aside, draining on kitchen paper if needed
Wilt 125g baby spinach in a dry non-stick pan over medium heat until just softened, then transfer to a plate to cool slightly
In a separate small saucepan, combine 1 teaspoon plain flour and 2 tablespoons milk, whisking until smooth
Place flour-milk mixture over low heat, stirring until it thickens into a roux (about 1 minute)
Take pan off heat, add 1 tablespoon mustard, ½ teaspoon Worcestershire sauce, and 200g grated cheese
Stir cheese mixture until smooth and mostly melted, return to gentle heat if needed (do not boil)
Once cheese is melted, remove from heat and let cool for 1 minute
Beat in 1 egg yolk quickly to prevent scrambling; mix until fully combined
Toast 4 slices of bread until golden on both sides under the grill or in a toaster
Lay wilted spinach evenly on top of each toast slice
Gently flake smoked haddock fillets and distribute over spinach
Spoon rarebit mixture over haddock on each slice, spreading to the edges
Place topped toasts on a baking tray and grill under high heat until rarebit is bubbling and golden (2–4 minutes)
While rarebits grill, arrange 80g salad leaves (spinach, watercress, rocket) on serving plates and top with orange slices
Drizzle salad lightly with olive oil just before serving
Plate rarebit slices alongside salad and serve immediately