In large skillet, heat oil over medium heat. Add onions and peppers; sauté until onions are softened (about 3-5 minutes). Stir in beans, tomatoes and 1 can enchilada sauce; set aside.
In medium bowl, combine avocados and sour cream; mash to desired consistency. Place ½ cup enchilada sauce in bottom of a 13x9-inch baking dish.
Top with 6 tortillas, half bean mixture, half mashed avocados and 1 cup cheese; repeat layers ending with tortillas, remaining enchilada sauce and remaining cheese.
Bake, covered, in a preheated 375°F oven 30 minutes; remove cover and continue baking 15 minutes or until cheese is bubbly and golden brown.