Rinse the cucumber, then coarsely chop. Rinse the tomatoes, remove the core, and cut into wedges. Peel and halve the onion. Cut into thin half-rings, and blanch briefly in boiling water. Transfer to an ice water bath, then drain well. Cut the feta cheese into 4 slices.
Mix all the ingredients for the dressing together. Mix in the tomatoes, cucumbers, olives, and thyme. Divide the mixture onto 4 dishes, then place the onions and cheese on top. Serve with fresh white bread, if you'd like.