Peel the cucumber and cut in half lengthways. Scoop out the seeds. Slice the cucumber into pieces roughly the thickness of a pound coin. Put in a colander and toss with 1 tsp of fine sea salt. Leave to stand in the sink for 30 minutes.
Preheat the oven to 180C/160C fan/gas 4. Cut the bread into cubes roughly 1cm square and put in a bowl. Pour over 3 tbsp of the oil and toss well together. Spread evenly over a baking sheet and bake in the oven for about 10-15 minutes (turning once or twice) or until dry and pale golden brown on the outside but faintly squidgy within. Leave to cool.
Score the base of each tomato with a small cross and place in a large bowl. Pour over enough just-boiled water to cover and leave to stand for 15-30 seconds. At this point the skins should begin to shrink back from the flesh. Lift out with a slotted spoon and leave until cool enough to handle.
Slip off the tomato skins and cut the tomatoes into quarters. Scoop out and discard the seeds of half of them and leave the seeds in the rest. Cut the tomato flesh into chunky dice, about 1cm square, and put in a large bowl. Scatter over the onion, garlic, capers, anchovies and lemon juice. Squeeze the cucumber in your hands over the sink to get rid of as much of the liquid as possible. Drop the cucumber into the bowl with the tomatoes and toss well together.
When ready to serve, stir the bread and 3 tbsp more olive oil into the tomato and cucumber mixture. Season with red wine vinegar and flaked sea salt. Spoon into flatish soup plates, sprinkle with parsley and add a few more slurps of the remaining olive oil. Serve.