First, combine the red onion, vinegar, sugar and salt in a bowl and set aside.
In another bowl, combine the tomatos with the olives, capers, olive oil, lemon zest and a pinch of salt.
Drain the onion mixture of any liquid and discard that liquid. Add the onion mixture to the tomato mixture.
Stir in the chopped parsley and check for seasoning. Season to taste with more salt & pepper if desired.