Wash the mussels in plenty of cold water and scrape off any barnacles with a knife. Discard any that don’t show signs of closing when squeezed gently, then pull out the tough fibrous beards protruding from between the closed shells.
Heat a large, heavy-based pan over a high heat. Add the mussels and two tablespoons of water, cover and shake around for 2-3 minutes, or until the mussels have just opened. Drain into a colander and leave until cool enough to handle. (Discard any mussels that do not open.)
Break away the empty, top half-shell from each mussel. Release the meat in the other shell, but leave it in place. Place them side by side on a serving platter.
For the dressing, put the chopped tomato into a mini food processor and blend until smooth. Add the vinegar, oil and some salt and pepper to taste and blend briefly once more to combine.
Tip into a small bowl and stir in the capers, cornichons, red onion, green pepper, red pepper and parsley. Spoon some of the dressing over each mussel and serve at room temperature.