Give the tomatoes, jalapeno, and cilantro a good rinse. Peel the onion and de-stem the tomatoes and jalapeno. I usually twist off and discard the bottom, thicker portions of the cilantro stems, but keep the upper portion of the stems that hold the leaves together.
Finely chop the 2 tomatoes, 1/2 onion, and jalapeno into 1/4" sized chunks. Finely dice the cilantro. For a milder version you can start by using only 1/2 of the jalapeno and adding more incrementally if you want more heat.
Combine the tomato, onion, jalapeno and cilantro in a mixing bowl. Add a heaping 1/4 teaspoon of salt and the juice of a single lime. Mix together well and taste for seasoning. Add additional salt and lime if necessary (I added more salt to this batch).
Serve immediately or store in an airtight container in the fridge.