Preheat oven to 350 degrees F. Make sauce: In a saucepan heat 3 tablespoons oil, add flour and cook, whisking, 1 minute. Add chili powder and cook 30 seconds. Stir in stock, tomato puree, oregano and cumin and bring to a boil. Reduce heat and simmer 15 minutes until flavors are well-blended. Season to taste with salt. Sautee onion, garlic and corn in skillet over medium high heat until soft. Combine half the cheese, chicken and onion mixture for filling. Using tongs dip each tortilla in sauce. On a plate fill with a generous spoonful of filling and roll up; place enchilada seam-side down in a baking dish and repeat until all ingredients are used up. Top with remaining sauce and cheese. Bake 30 minutes. To serve, top with sour cream and scallions.