First, roughly chop up the onion, tomatoes, jalapeño (remove the seeds and membrane if you don't want it to be too spicy), and cilantro.
2 cups fresh tomatoes, ½ onion, 1 jalapeño, 1 cup cilantro
If your tomatoes seem pretty watery, you can cut them up, lightly salt them, and let them sit on a towel for 15 - 30 minutes. Pat them dry, then use for your salsa. The salt will draw out the excess water in the tomatoes and will keep your salsa from getting too watered down. (If you use this method, use less salt in your salsa!)
Next, add your tomatoes, jalapeño, most of your cilantro, and your onion to your blender or food processor. Give it a good, rough blend.
Stir everything together and scrape down the sides of the blender, then add the rest of the cilantro, the salt, the lime, and the cumin. (If you salted the tomatoes, use less salt now.)
⅛ teaspoon cumin, ¾ teaspoon kosher salt, ½ lime
Blend to your desired consistency and serve immediately with tortillas chips.