Cook first 5 ingredients in a large nonstick skillet over medium-high heat, stirring often, 10 to 12 minutes or until vegetables are tender and beef crumbles and is no longer pink.
Stir in salsa and next 3 ingredients. Cook 5 to 10 minutes or until thoroughly heated.
Layer 1 cup beef mixture, 2 tortillas (overlapping edges), and 1/2 cup cheese in a lightly greased 7- x 5- x 1 1/2-inch baking dish. Repeat layers once. Repeat procedure with 3 additional 7- x 5- x 1 1/2-inch baking dishes. Cover dishes with nonstick aluminum foil.
Bake, covered, at 350° for 30 minutes; uncover and bake 5 more minutes or until bubbly. Let stand 10 minutes before serving. Serve with desired toppings.
Note: To use a lightly greased 13- x 9-inch baking dish, prepare recipe through Step Omit 4 corn tortillas. Layer one-third beef mixture, 6 tortillas (overlapping edges), and 2 cups cheese. Repeat layers with one-third beef mixture, remaining 6 tortillas, remaining beef mixture, and ending with remaining 2 cups cheese. Bake, covered, at 350° for 40 minutes; uncover and bake 10 more minutes or until bubbly. Let stand 10 minutes before serving.