Pour 300ml skimmed milk into a deep frying pan or a wok. Add a bay leaf and the haddock fillets. Pop on the lid. Bring to a gentle boil. Turn the heat down to low. Gently simmer for 2-3 mins till the haddock is opaque.
Lift the haddock out of the pan. Pop on a plate. Flake with a fork and pop to one side. Strain the milk into a jug or bowl through a fine metal sieve. Wipe the pan clean with kitchen paper. Heat your oven to 180°C/Fan 160°C/Gas 4.
Trim the leek. Slice a cross into the top of it so you can open out the layers. Rinse. Finely slice it. Peel and grate or crush the garlic clove. Rinse the baby leaf spinach. Rinse and halve the cherry tomatoes.
Pop the leeks into the frying pan or wok with 2 tbsp cold water. Season. Cook over a low heat for 5 mins till the leek is soft. If it looks like it might burn, add a splash more water.
Add the garlic to the pan. Cook and stir for 1 min. Measure out 1 tbsp cornflour. Add to the pan. Cook and stir for 1 min. Slowly pour in the milk, stirring while you add it, till you have a smooth sauce. Simmer for 2 mins.
Stir the spinach into the sauce so it wilts. Add the cherry tomatoes and fish. Spoon into an ovenproof dish. Rinse the celeriac and halve it. Peel one half (slice off any roots). Coarsely grate. Pop in a bowl. Rinse the chervil. Pat dry. Finely chop.
Add the chervil to the celeriac with 1 tbsp olive oil and some salt and pepper. Mix. Spread the celeriac over the top of the fish pie. Bake in the oven for 30-35 mins till the pie is lightly golden and bubbling. Serve straight away.