Place the tomatoes in a medium saucepan and cover with water. Set over high heat, bring to a boil, and let simmer over medium heat for 7 to 8 minutes, until completely cooked and softened. Using a slotted spoon, transfer the tomatoes to the jar of a blender or food processor and puree just until coarsely chopped. Or you may chop them with a knife, but don't discard the juices, skin or seeds. Set aside.
Heat the oil in a medium saucepan set over medium heat. Once hot, add the chiles, cook for 3 minutes, stirring occasionally, until they start to change color. Add the onion and continue to cook until softened and beginning to brown, about 7 to 8 minutes. Incorporate the chopped tomatoes, add the salt, and cook for 5 to 6 minutes more, until the color becomes deeper and it thickens. Taste for salt and add more if need be.