Heat the oil in a large, heavy nonstick skillet or a wide saucepan over medium-high heat, and add all of the tomato puree, and cook, stirring, for about five to ten minutes, until the sauce thickens then season to taste with salt. Turn the heat to low, and simmer, stirring often, for about 5 minutes. Stir in the chicken and heat through. Stir in the tortilla chips, stir together for a half minute, and remove from the heat. Sprinkle the cheese, cilantro and sliced onion over the top, and serve.