In a large skillet over medium heat, combine first 6 ingredients, bringing the mixture to a boil. Reduce heat and simmer uncovered for 5 minutes. Stir in beans.
Spread 1/3 of the bean mixture over the bottom of a 13x9-inch pan. Top that with half of the tortillas, overlapping as necessary and half of the cheese. Add another 1/3 of the bean mixture, then remaining tortillas and bean mixture.
Cover and bake at 350°F for 30 minutes or until heated through. Sprinkle with remaining cheese and let stand for 10 minutes. Garnish with shredded lettuce and chopped tomatoes.
Serve by topping with the sliced avocado or guacamole.