In a skillet heat oil and sauté the onions until translucent, about 5 minutes. Add the garlic and cook for 1 minute longer. Stir in tomatoes, chiles, oregano, salt, and sugar. Simmer for 15 minutes, stirring occasionally.
Combine 1/2 cup of the sauce with the chicken, 1/2 cup of shredded cheese, and 1/4 cup of sour cream; set aside.
Spread about half of the remaining sauce in a 9-by-13-inch baking dish.
Add about 1/4 cup of the chicken mixture to a tortilla and roll it up; place it in the baking dish. Repeat with the remaining tortillas and chicken mixture.
Pour the remaining sauce over tortillas.
Sprinkle with the cheese.
Bake for 20 to 25 minutes or until bubbly. Serve chicken enchiladas with dollops of sour cream or guacamole and garnish with sliced olives if desired.