Remove and discard the cores from the tomatoes and the stem from the jalapeño. Place tomatoes and jalapeño in a small saucepan then add just enough water to cover. Bring to boil over medium-high heat, then reduce the heat to medium-low and simmer for approximately 15 minutes or until the skin has broken away from the tomatoes.
Drain tomatoes then place in a blender along with the jalapeño. Pulse until roughly chopped.
Add salt, chopped onion and cilantro. Puree until smooth. (This is not a chunky salsa.)
Taste salsa then and add additional salt, onion or cilantro if desired. If the salsa is too spicy, boil another tomato or two then puree it into the salsa to adjust the heat. If you add more tomato, you may also want to add a bit more onion and cilantro, and it will certainly need a bit more salt. Serve with your favorite tacos, burritos, quesadillas or tortilla chips and enjoy!
Wear disposable gloves while handling the jalapeños to avoid getting those spicy juices in your eyes. Ouch!