It is difficult to make this particular sauce for fewer than eight to ten people, but it keeps well for several days and even improves with keeping.
1onion weighing about 150 g (5½ oz)
3tomatoes weighing about 400 g (14 oz) altogether
1 small cucumber weighing about 200 g (7 oz)
1sweet red pepper weighing about 150 g (5½ oz)
100g (3½oz) pickled gherkins
100g (3½oz) pickled capers
1teaspoon chopped tarragon
50g (1¾oz) chopped parsley
200ml (⅓pint) olive oil (13tablespoons)
3generous tablespoons wine vinegar
Peel the onion and the cucumber. Cut the latter in half lengthways and remove the seeds with a teaspoon. Remove the stalks from the pepper and shake out any remaining seeds. Remove the stalks from the tomatoes, cut them in half and press to squeeze out the seeds and juice.
Cut all the vegetables into small dice the size of very small peas and place in a stoneware or porcelain bowl. Add the gherkins cut into pea-sized dice, together with the capers, the chopped tarragon and parsley, the mustard, the olive oil and the wine vinegar. Season generously with salt and pepper. Mix very thoroughly with a wooden spatula, cover the bowl and keep in a cold place (but not in the refrigerator) for 24 hours before serving.