Place the tomatoes under the broiler anywhere from 6 to 10 minutes, until completely charred and mushy. Alternatively, you can char them on a hot comal or skillet set over medium heat, or on a grill. Remove from heat and set aside to cool. Once cool enough to handle, chop, without discarding the juices or seeds.
In a large skillet or saucepan, heat the oil over medium heat. Once hot, add the onion, and cook stirring often until softened. Add the garlic and cook for less than a minute until fragrant and lightly browned, but don’t let it burn. Incorporate the chile (add 2 for a more spicy salsa), stir, and cook for a minute. Raise the heat to high. Add the tomatoes, with all of the seeds and juices, and cook for about 6 to 8 minutes.
Pour in the chicken broth and salt, stir, and cook for another 8 to 10 minutes. Add more broth if needed. The salsa should have a thick, yet still very wet, consistency.