Remove the tough stalks from the borage. Add the leaves to a blender with the bread, cucumber, olive oil, mint, garlic and hazelnuts, and blitz until smooth. Add the yoghurt and 2 tbsp of the vinegar and blend again. Taste and add the other tablespoon of vinegar if required, along with the Tabasco and sugar. This should be quite a thick soup, but thin it with a little water if you prefer. Season to taste and leave to chill until serving.