Put the spring onion whites, milk and flour in a saucepan together. Bring to a simmer, whisking continuously, until the sauce has thickened and become smooth. Turn the heat down, add the drained potatoes and parsnips, the lemon zest and half the juice, and cook gently for 5 mins, stirring occasionally. Stir in the spring onion greens, fish and parsley, and taste for seasoning – it will need plenty of pepper, some salt and maybe more lemon juice from the leftover half. Divide between two shallow bowls, serve with chunks of crusty bread and enjoy.