Heat oil in a skillet over medium heat. Add onions, and cook stirring often for about 4 minutes, until they have softened but not browned. Stir in the chiles, cook for another minute. Incorporate the chopped tomatoes and let the mix cook and season, stirring often, for about 5 minutes (I let them cook about 4 to 5 minutes more until tomatoes are thoroughly cooked, mushy and their red color has deepened).