In a large skillet over medium-high heat, cook diced onion and brown ground beef.
Drain off excess grease, then stir in contents of taco seasoning packet. Set aside.
In a 9x13 casserole dish, spread a thin layer of enchilada sauce evenly on the bottom.
Layer tortilla halves, so straight edges line up along the outer edge of the dish. Continue layering to fill any gaps (it’s ok if there is some overlap).
Continue layering the lasagna in the following order (saving enough of each ingredient to form a second layer): refried beans, enchilada sauce, beef and onion mixture, shredded cheese and diced tomatoes. You may then sprinkle on fresh cilantro and/or black olives if you wish before starting the next layer.
With the second half of ingredients, start the next layer as follows: tortillas, refried beans, enchilada sauce, beef and onion mixture, shredded cheese and diced tomatoes (optional fresh cilantro and/or black olives).
Top with a final layer that includes tortillas, a light layer of enchilada sauce and shredded cheese.
Bake in preheated oven for 30-35 minutes or until cheese is melted and tortillas have slightly crisped.
Allow lasagna to cool for 5 minutes before serving.
Serve with your choice of toppings: sour cream, guacamole, salsa fresh cilantro, shredded lettuce, fresh tomatoes, etc.