1. Preheat the oven to 190°C/gas mark 5. Lightly grease four individual gratin dishes with a little oil.
2. Put the smoked haddock into a medium pot along with the chopped onion, water and a pinch of ground white pepper. Bring to the boil, then reduce the heat and simmer for 3 to 4 minutes. Strain the water off the haddock.
3. In a separate medium pot, bring the cream to the boil, then add the spinach and remove from the heat.
4. Divide the smoked haddock between the gratin dishes. Spoon the creamed spinach over the haddock and mix together gently. Sprinkle the grated cheese on top.
5. Bake in the oven for 15 minutes, until the haddock is cooked through and the gratin is golden brown. Serve straight away.