Melt the butter and add flour and make a roux (paste). Gradually add the warmed milk until smooth. Allow to simmer and pass through a fine strainer.
Make the mash potato.
Slice the leek and fry in butter in a thick bottomed pan until soft. Add the white sauce and bring up to a simmer. Add the cheese and cook for 10-15 minutes.
Poach the haddock in milk for 7 minutes.
Poach the eggs.
Just before serving lightly steam the spinach until just wilted.