Slice the mango into three portions (the center one will have the hard seed). Take the "side" portions - with the skin and all. Make a criss-cross pattern and then use a spoon to scoop out nice clean mango bits.
Dice red onion and jalapeno. Remove the seeds and chop up tomatoes and cilantro into small pieces. Add to a mixing bowl with the mangoes, and give it a quick stir to combine.