Add the onion, garlic, jalapeno peppers, and cilantro to a food processor, and pulse until you have finely chopped all of those ingredients.
Scoop that into a small bowl, and add in the lemon juice, mayonnaise, mustard, and salt.
Give a really good stir. If you want the mayonnaise to be a bit thinner, add the water, and stir again. Taste it. Thank me later.
This mayonnaise is pretty darn phenomenal. My wife, who never sways toward mayonnaise loved it as well. In a nutshell, everything balances out, and it is just the perfect sauce, or condiment if you will. Simply store in an air tight container in your refrigerator and use it on everything from baked chicken, to tacos, to sandwiches. Heck, I even took a little spoonful of it, and ate it by itself, it is that good. Hope you enjoy.