Preheat your oven to 350°F (175°C). Lightly grease a 9-by-13-inch baking dish to prevent sticking and aid even browning.
In a large bowl, combine shredded chicken, Rotel with its juice, cream of mushroom soup, cream of chicken soup, milk, and sour cream. Stir until everything is smoothly blended. The mixture should be creamy but not too runny.
Stir in half of the shredded cheddar cheese and the finely chopped onion. If you like a bit more warmth, add the taco seasoning and mix well.
Fold in half of the tortilla strips, leaving a generous amount to layer on top later. This helps to build texture and ensure every bite has some tortilla goodness.
Spread the mixture into the prepared casserole dish in an even layer. Scatter the remaining tortilla strips over the top, followed by the rest of the shredded cheese.
Bake for 25–30 minutes, until the edges are bubbly and the cheese is melted and lightly golden. If you prefer a crisper top, switch to a broil for the last 2–3 minutes, watching closely to prevent burning.
Remove from the oven and let the casserole rest for 5–10 minutes before serving. This helps the sauce thicken a bit and makes scooping easier.
Serve warm, with a quick tossing of crushed tortilla chips on top for extra crunch if you like, and a side of fresh pico de gallo or a quick green salad to balance the richness.