To Make Chicken Mixture: In a medium skillet heat oil. Add onion and saute until translucent. Add tomato, chicken and salt and pepper to taste. Cook until heated through. Set aside.
To Make Cheese Mixture: Mix together rancherito cheese and Romano cheese.
Pour tomato sauce into a shallow dish or bowl. In a separate medium skillet, heat oil. One by one, dip corn tortillas in tomato sauce, covering both sides. Then slip carefully into hot oil in skillet. Fry for only about 30 seconds on each side. Set on clean dish and fill with chicken mix and a sprinkle of cheese mix. Roll up. Repeat until all tortillas are filled OR filling runs out, whichever comes first. Top with shredded lettuce, diced tomato and sour cream and serve.