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.
Combine cheese, chicken and onion for filling. Heat remaining 1/2 cup oil in a skillet until hot. Using tongs dip in tortillas, one at a time, to soften, and drain on paper towels. 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. Bake 30 minutes. To serve, top with sour cream and scallions.