Preheat your oven to 350°F (175°C).

In a large mixing bowl, combine the cooked chicken, finely chopped onion, and half of the queso cheese. Stir until evenly coated.

Take a tortilla and spoon a generous portion of the chicken filling along one edge. Roll tightly and place seam side down in a greased baking dish. Repeat until all filling is used.

In a medium saucepan, combine the creamy queso sauce, diced tomatoes, and chili powder. Mix until smooth and warmed through, about 5-7 minutes.

Pour the sauce evenly over the assembled enchiladas in the baking dish. Sprinkle the remaining queso cheese on top.

Bake in the preheated oven for 20-25 minutes until cheese is melted and lightly browned.

Remove from oven, let cool for a few minutes, and top with sour cream, fresh cilantro, and avocado slices.

Serve on plates, pouring any remaining sauce over the enchiladas.