Preheat oven to 350ºF.
In a saucepan, combine the soup, milk, and the ¾ of the cheese, reserving the remaining ¼ for topping the casserole. Once the cheese is melted, stir in the green chiles.
In a skillet, brown the beef with the onions. Add in the oregano, garlic powder and salt.
In a well greased 9×13 casserole dish, layer half of the tortilla strips. Spread ½ of the beef mixture on top, followed by ½ of the cheese mixture. Repeat layers. Sprinkle reserved cheese on top.
Bake in preheated oven until heated through and cheese is melted, about 30 minutes. Let the casserole sit for 10-15 minutes before serving if you want the slices to hold together.
