In frypan, cook chicken and onion together in oil over medium-high heat until chicken is just done. Divide cooked chicken evenly between 8 tortillas; add 1 1/2 tablespoons cheese to each tortilla. Roll enchiladas and place seam-side down in 9x13" baking dish that has been lightly sprayed with no-stick cooking spray. Melt butter in a medium saucepan; stir in flour to make a roux; stir and cook until bubbly; gradually whisk in chicken broth then bring to boiling, stirring frequently. Remove from heat; stir in sour cream and green chiles; pour sauce evenly over enchiladas. Top with remaining 3/4 cup cheese (baking dish may be double-wrapped and frozen at this point) and bake at 400° for 20 minutes until cheese is melted and sauce near edges of baking dish is bubbly.