1.
Wash the cockles and mussels in a few changes of cold water, scrubbing to remove any beards still attached to the shells. Discard any that are damaged, or opened ones that do not close when tapped. Place the remaining shellfish in a large plastic bowl, cover with cold water and set aside to soak for 1 hour.
2.
Drain the shellfish thoroughly and place in a stockpot or large saucepan. Add the wine, cover and cook over a high heat for 2-3 minutes until all the shells have opened.
3.
Strain off the cooking liquid and reserve. Discard any shellfish that have not opened. Pick half the shellfish from the shells and leave those remaining in their shells.
4.
Heat the vegetable oil in a saucepan, add the onion, celery, red pepper, carrot and sage and cook over a gentle heat until softened. Add the canned tomatoes and bring to the boil, stirring frequently. Cook for 3 minutes, then add the reserved cooking liquid from the shellfish. Return to the boil and simmer for a further 3 minutes, stirring frequently.
5.
Stir the cockles and mussels into the sauce and add salt and pepper to taste. Stir in the parsley and serve hot with herbed scones, pasta or rice.