In a large skillet, coated with non-stick spray, add your diced onions and cook for 4-5 minutes. Add your chicken and continue to cook until the chicken is no longer pink and cooked through, approximately 6-8 minutes. Add the jar of enchilada sauce to the skillet and simmer for 2-3 minutes.

Meanwhile, in a saucepan over medium heat, melt the butter. Add the garlic, onion powder, salt and flour to the pan and mix well. Immediately add the milk and whisk until well combined. Gradually add the cheese, ensuring it is melted with each addition. Simmer on low until ready to serve, being sure to stir every few minutes so it doesn't burn on the bottom.

Spray each of the tortillas with non-stick spray on both sides and heat them in a separate skillet over high heat for one minute on each side.

Spread 1/4 c. chicken mixture over a tortilla and top with another tortilla. Continue this process three more times and finish it off with a tortilla on top. Repeat to assemble the second stack.

Top each stack with a generous serving of queso sauce along with tomatoes, cilantro and avocado. To serve, cut large wedges from each stack.