Warm the oat milk gently in a deep pan with the bay leaf and peppercorns. Carefully place the haddock into the hot milk and poach for 2–3 minutes until it’s just cooked.
Remove the fish from the milk, allow to cool a little, then remove any skin and use your fingers to flake the fish into large chunks.
Strain the milk you used to cook the fish through a sieve into a clean jug or bowl and set aside.
Heat a large non-stick pan and add the onions, leek and celery. Cover and cook over a low heat for around 5 minutes until the veg are soft.
Add the stock, strained oat milk and potato to onion mix and simmer until the potato is cooked, around 15 minutes. Once the potato is cooked, add most of the fish to the pan, reserving some for garnish.
Remove half the soup from the pan using a ladle, and blitz in a food processor or blender until smooth, then return to the pan with the remaining soup and heat gently. Season with salt, pepper and lemon juice, to taste.
To serve, ladle the soup into 4 bowls and garnish with the remaining flaked fish, chives and a drizzle of the rapeseed oil.