Rinse the cucumber thoroughly, dry it. Halve lengthways, scrape out the stones with a teaspoon. Cut the cucumber halves into narrow crescents. Wash the blueberries and pat dry. Clean and wash the spring onions and cut into rings.
For the dressing, mix together the fruit vinegar, salt, pepper, cayenne pepper and sugar and beat in the olive oil.
Mix the cucumber, spring onion, blueberries and mint with the dressing, carefully fold in the crumbled feta, let the salad steep a little, then season to taste again.