Preheat your oven to 350°F (175°C).
In a skillet, heat olive oil over medium heat and sauté the chopped onion until translucent.
In a large mixing bowl, combine the cooked chicken, cream of chicken soup, cream of mushroom soup, diced tomatoes, and milk.
Add the sautéed onions to the mixture and season with salt and pepper.
In a greased baking dish, layer half of the tortilla strips at the bottom.
Spread half of the chicken mixture over the tortilla strips.
Sprinkle half of the shredded cheddar cheese on top.
Repeat the layers with the remaining tortilla strips, chicken mixture, and cheese.
Cover the dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 10 minutes until the cheese is bubbly and golden brown.