Boil water, salt and pepper in a pan. After it boils, add shrimp. Let it boil for 3 minutes (or until shrimp becomes lightly red) and remove from heat. Be careful not to over cook - shrimp will become a chewy. Pour out water and let shrimp cool.
Cut the onion into long thin strips -julienne style. Place them in a bowl with water and 2 tablespoons of salt and let it settle for 10 minutes. Then gently rinse with water. This will remove the bitter bite from the onion and make it sweeter.
In a large bowl, mix the onion, tomatoes, orange and lemon juice, mustard, cilantro (or parsley), oil, salt and pepper. Once the shrimp has cooled, add shrimp and mix.
Serve with fried plantains (fried green bananas) or plantain chips.