Slice each side of the mango just before the seed. Using a sharp knife slice flesh vertically, without breaking the skin. Scoop out the slices and dice. Chop the onion, cilantro and jalapeño pepper.
Combine everything in a bowl, add salt and lime juice to taste. It can be made at any time, but it is best to make one day in advance. You can top this mango salsa with some cayenne pepper for extra heat. Add more citrusy flavor with a little bit of lime zest.