Preheat the oven to 350°F. Combine the onion, garlic, and 2 tablespoons of oil in a 1 quart saucepan. Stirring occasionally, cook over medium heat until the onion turns translucent about 3 minutes. Add the tomato sauce, oregano, and salt. Reduce the heat to low and cook, uncovered, for 15 minutes. Meanwhile, season the chicken with the garlic salt and pepper and put it in a medium nonstick skillet, along with the remaining 2 tbsp oil. Heat for about 10 minutes over medium low heat. Remove the meat to a bowl, and warm the tortillas in the same pan for about 15 seconds a side. Divide the chicken and the chilies among the tortillas. Roll the tortillas up and place them in a 9x13 baking dish that has been buttered. Cover with the tomato sauce, and sprinkle with the grated cheese. Cover and bake for about 15 minutes, until the cheese is melted and bubbly, top with sour cream, if desired.