Heat oil in a large nonstick skillet over medium heat until hot. Add bell pepper and next 3 ingredients; sauté 5 minutes or until vegetables are soft. Stir in cumin, and cook 1 minute. Stir in flour; cook 3 minutes. Add chicken broth and sour cream; simmer 5 minutes, stirring frequently. Add chicken, and stir to blend.
Soften tortillas by layering them between damp paper towels and heating in microwave, in several batches, at HIGH for 30 seconds.
Arrange 4 tortillas in a lightly greased 3-qt. baking dish. Top with one-third of chicken mixture (about 2 cups), sprinkle with 1 cup cheese, and cover with 4 more tortillas. Repeat layers with another one-third of chicken mixture, 1 cup cheese, 4 tortillas, and ending with remaining chicken and 1 cup cheese. Cover with aluminum foil, and refrigerate up to 2 days.
Bake, uncovered, at 350° for 35 to 45 minutes or until bubbling and browned.
Note: You can also freeze this casserole. Follow the make-ahead directions above but freeze after preparing. When ready to cook it, bake foil-covered frozen casserole at 350° for 1 hour and 15 minutes or until thoroughly heated.