Heat the oil in a large skillet set over medium heat. Add the onion and cook until softened but not brown, about 5 minutes. Reduce the heat to medium-low, then add the chiles and cook for one minute. Add the tomatoes and cook until all liquid has reduced.
Beat the eggs with the salt, just enough to combine the whites and yolks. Add the beaten eggs to the skillet and scramble until they are as done as you like. Taste for salt, then scoop them into warm tortillas and garnish with cilantro and diced avocado.