Position blade in food processor, and add 1/2 cup onion and next 3 ingredients; process. Saute chili powder and oregano in 2 tablespoons oil in a skillet 3 minutes. Add tomato mixture; simmer 5 minutes. Combine cornstarch and 2 tablespoons broth. Add remaining broth and cream to tomato mixture; cook 2 minutes. Add cornstarch mixture; cook until thickened. Stir in salt and pepper.
Saute remaining 1 cup onion and minced garlic in 2 tablespoons hot oil in a skillet over medium-high heat until tender. Add chicken; cook until lightly browned. Stir in 1 cup tomato mixture.
Pour 3 1/2 cups oil into a skillet. Fry tortillas, one at a time, over medium-high heat 8 seconds on each side. Drain on paper towels. Brush one side with tomato mixture. Spoon about 1/4 cup chicken mixture down center. Roll tortilla; place, seam side down, in a greased 13- x 9- x 2-inch baking dish. Repeat with remaining tortillas. Pour remaining tomato mixture over tortillas; sprinkle with cheese. Bake, uncovered, at 375° for 15 minutes. Top with sour cream and green onions.