I've been making this soup for years. It's lovely and sweet, with loads of flavour and stands up well to other ingredients. If you can't get any haddock, substitute sustainable cod or pollack.
50g butter, unsalted
4 tbsp olive oil
1.5kg white onions, thinly sliced
2 sprigs of thyme
Pinch of rock salt
600-800ml vegetable stock or water
Milk to cover
1 clove garlic, crushed
1 tbsp flat leaf parsley
2 tbsps double cream
Slice of dried toast, broken into pieces
Melt the butter in a pan over a low heat and add 2 tbsps olive oil. When hot, add the onions, a sprig of thyme and salt.
Cover with a lid to allow the onions to cook quickly but without colouring and cook gently for five to six minutes until softened.
Remove the lid and add the stock. Continue to simmer for around eight to 10 minutes until the onions are cooked through.
While the onions are cooking place the haddock in a small pan and cover with the milk, the second sprig of thyme and crushed garlic.
Bring to a simmer for three minutes, remove from the heat, allow to cool then flake the haddock .
Transfer the onion mixture into a food processor or blender with another splash of oil.
Blitz into a purée, check the consistency and seasoning then mix with the haddock and parsley. Serve with the toast, cream and a drizzle of olive oil on top.
• Angela Hartnett is chef patron at Murano restaurant and consults at the Whitechapel Gallery and Dining Room, London. Twitter.com/angelahartnett