Peel onion and cut into rings. Wash and clean the tomatoes and cut them into pieces. Clean, wash and cut lettuce into strips. Dice cheese. Remove avocado flesh from the skin and cut into fine strips.
Mix vinegar and stock, season with pepper and sugar. Wash basil, dab dry and cut into fine strips, except for 1 leaf for garnishing. Mix onion, tomatoes, olives, lettuce, basil strips and feta cheese with the vinaigrette.
Arrange in a bowl, sprinkle with pepper and garnish with basil.