Preheat oven at 350*.In large sauce pan, melt butter over medium heat. Gradually stir in flour until completely blended and smooth. Add chicken broth and stir until smooth. Fold in Sour Cream and stir until smooth. Add Rotel, stir. Remove from heat.
Cover bottom of 9x13 baking dish with tortillas. Layer chicken, onions and 1/2 of the cheese. Pour half of the sour cream sauce over chicken. cover with tortillas. Add remaining Sauce and top with cheese.