Step by step

1. Put the haddock, milk, onion and peppercorns in a deep, lidded frying pan. Bring the milk to a simmer then put on the lid and turn off the heat. Leave for 10 minutes, then take out the haddock and flake into large chunks, discarding any skin and bones. Strain the milk into a jug.
2. Drop the potato slices into boiling water and cook for 7-10 minutes or until tender. Drain well.
3. Wilt the spinach in a large pan over a medium heat with a splash of water. Tip into a colander and cool, then use your hands to squeeze out as much liquid as possible. Tip onto doubled-up kitchen paper and pat as much water out as you can. This is important to prevent a puddle forming at the bottom of the dish. Chop the spinach then toss with lots of seasoning.
4. Heat the butter in a pan and, when foaming, stir in the flour. Stir for 3-4 minutes or until the mix has a nutty aroma. Gradually whisk in the strained milk a little at a time until you have a sauce. Simmer for 3-4 minutes then tip in the cheeses and stir until melted.
5. Heat the oven to 220C/fan 200C/gas 7. Butter a large ceramic baking dish and put the spinach in the bottom. Top with the haddock chunks and potatoes, then spoon over the sauce. Bake for 15-20 minutes or until the sauce is bubbling and the top golden. Serve with salad, if you like.