Step 1 Boil 2 cups water in a large pot then add one-third of the washed mussels, cover and heat until shells open 5mm. Transfer promptly, with tongs, into a large bowl. Repeat until all mussels have opened. Save the liquid.

Step 2 Heat the first measure of oil in a non-stick pan and add onions. Cook without browning, stirring occasionally, until softened. Add carrot, celery and potatoes. Strain the mussel cooking liquid into the pan of vegetables. Simmer until vegetables are tender.

Step 3 Take mussels from shells, discarding beards. Dice mussel flesh in 1cm cubes and set aside.

Step 4 Heat the second measure of oil in the cleaned mussel pot then stir in flour. Add milk and bring to the boil, stirring or whisking constantly, then add 2 cups water and boil again, still stirring. Remove from heat. Add cooked vegetables with their cooking liquid and season carefully to taste.

Step 5 When ready to serve chowder, heat, stirring constantly, until very hot. Add two-thirds of the mussels and any liquid then pour into large bowls. Top with the remaining mussels and chopped herbs.