Chop the onion and tomato into small cubes. I don't seed the tomatoes, I use all of it. Place it in a plastic or glass container. Don't use metal - the acid in the tomato will discolor the bowl and make the salsa taste funny. Mix together and look to see that it looks evenly red and white. If not, add more of the ingredient that looks like there is less of it.
BE SURE TO USE GLOVES IF YOU HAVE NEVER HANDLED JALAPENOS BEFORE! YOU WILL FORGET AND TOUCH YOUR EYE OR NOSE OR LIPS (OR ELSEWHERE) AND IT WILL HURT! Chop the jalapeno - if you don't like very spicy food, remove the seeds and the membranes from the jalapeno(s). I leave them in and I use about 4-5 jalapenos, but I like spicy food. Add to bowl.
Chop the cilantro finely and try to use more of the leaf part. It is a bear to chop but don't use a food processor as it tends to bruise the leaves. I just pull the leaves off and roll them up and chop them. In the end, just do what is easiest for you! Add to bowl.
Mix all of these ingredients with chopped garlic - I use the kind already chopped in the jar in the produce section (yep folks, I cheat!). Add lime juice (be sure not to include the seeds) and mix well. Sometimes you want it a little "soupy-er" and you can add a little El Pato in the yellow can. It can be found in the ethnic aisle in the supermarket. It is Mexican tomato sauce. The one in the green can is jalapeno - unless you wish to set fire to some unsuspecting person, don't use this one! 'kay?
If possible, let this sit overnight but at least for 2-3 hours. Serve with chips, etc. Really good and really easy! Enjoy!