Prepare the marinated onions in advance (they need to rest in the fridge for at least 20 minutes, ideally 45): mix the finely sliced red onions with the vinegar and the lemon juice. Add a pinch of sugar if you want to balance the strong flavour on the onions and the acidity of the mix vinegar/lemon.
Chop all the vegetables and the cheese.
Mix the ingredients in a large bowl or combine the salad directly into the individual serving plates.
Add the oil, fresh mint leaves and black pepper just before serving.