Peel and dice the peaches and add to a medium sized bowl. Very finely chop the red onion and add to the bowl with the peaches. Take the jalapeño and remove the stem, seeds, and ribs. Mince half of the pepper (or all of it, depending on how spicy you want it!) and add it to the bowl. Lastly, take a handful of cilantro and roughly chop it and add it to the bowl.
Using a spoon, mix all the salsa ingredients together until well combined. Add the juice of one lime (and the zest as well, for a stronger lime flavor) and a pinch or two of kosher salt, to taste. Serve immediately.