Prepare the mussels by washing them in cold water and removing any beards. Discard any that are damaged. Omit this step and step 2 if you are using pre-cooked mussels.
Bring 250ml water to the boil in a large pan. Add the mussels, place a lid on and allow the mussels to steam away in the saucepan for about 8 – 9 minutes or until all the shells have opened. Remove the mussels from their shells, set aside and discard any mussels that have not opened.
Heat the olive oil in a pan and add the onion, celery and leek. Sauté until softened.
Add the milk, bay leaf and potatoes then bring to the boil and simmer for about 10 minutes until the potato has softened.
Mix the corn flour and cold water together to form a paste then stir into the milk and keep stirring until it thickens.
Add the mussels to the mixture, season with salt and pepper and a little nutmeg.
Divide the chowder between two bowls and serve sprinkled with the chopped parsley.