Preheat oven to 350 degrees. Coat a 13x9 baking dish with nonstick spray.
In a saucepan, melt butter. Saute onion and garlic in butter until tender. Add tomatoes, tomato sauce, chiles, sugar, cumin, salt, and oregano. Simmer for about 20 minutes until heated through.
Soften tortillas in a warm skillet, then dip each side of tortilla in sauce. Fill enchiladas with about 1/3 c. of chicken and sprinkle with 2T. of cheese. Roll and place seam side down in prepared dish. Repeat with remaining tortillas.
Blend sour cream with remaining enchiladas sauce. Pour over enchiladas in dish then sprinkle with remaining cheese.
Bake uncovered for 40-50 minutes until sauce is bubbly and cheese is melted.