Salsa criolla (Peruvian pickled onions) is an essential accompaniment to all sorts of dishes. You can pair it up with any rich and/or dish that is on the dry side and could use some a bit of juicy tangy zing.
Optional but recommended: soak onion in cold water and a pinch of salt for a few hours, then drain in a strainer.
Add vinegar, lime juice, salt to taste, finely chopped chilies (if using) and finely chopped coriander leaves. Chill in the fridge until ready to serve.