Cook the onion gently in 1 tablespoon of olive oil for 8-10 minutes till very soft. Meanwhile, in a second pan, stew the leeks for 15-20 minutes, adding the potatoes for the last 5 minutes
Add celery, capers, sultanas, tomato, wine vinegar and sugar to the onion and cook for a further 15 minutes till reduced to about 50%
Check the seasoning of the tomato sauce and add salt and pepper as necessary
Pour the sauce over the leek and potato mixture and stir through.
Cover and leave to blend at room temperature for at least 30 minutes or till cool.
Store in the fridge.
Warm through to serve hot, or use cold garnished with toasted pine nuts