For sauce:
Bring water to the boil in a saucepan,remove tomato core and cut a small cross on the bottom of each one.
Gently place tomatoes in boiling water for about 30 seconds then remove and place in iced water. Take tomatoes out,peel then roughly dice and put aside.
Heat oil in a heavy-based saucepan over a low to medium heat,add onion and garlic and cook for about 4 minutes until softened but not coloured.
Add sherry and simmer for 1 minute until it has reduced a little. Add tomato and cook for 5 minutes until soft,then add sardines and cook for a further 5 minutes.
Remove sauce from heat,puree in a blender until smooth.
For toast:
Drain sardines and place in a bowl. Add tomato,shallot,parsley,lemon juice and olive oil.
Season and mix with a spoon but don't break the fish up too much or there won't be enough texture in your sardine salad.
To serve,grill the sourdough slices,place one each on four plates,then spread a spoonful of sardine sauce over each slice.
Top toast with watercress and spoon the sardine salad evenly on top. This dish is a fantastic light supper.
* The sauce is also a great addition to poached fish or pasta with cherry tomatoes and grilled zucchini.