Soak the onion slices in salt water for about 10 minutes, rinse well and drain. This will help remove the bitterness from the onions.
Mix all the ingredients together in large bowl and let it marinate in the fridge for 1 to 2 hours.
When ready to serve, give it another mix and taste. Add additional salt, lime juice, orange juice or other ingredients to your taste.
Serve with your choice of garnishes: patacones (tostones) or chifles (thin platain chips), tostado corn nuts (or popcorn), aji hot sauce, avocado slices, etc.
Notes
*You can add some of the broth from cooking the shrimp to the ceviche marinade for extra shrimp flavor. You can also blend the shrimp peels/heads with a fresh tomato and some broth - strain it and use this mix instead of the ketchup.