Pre-heat oven to 350ºF.
In a saucepan, combine the soup, milk, and 3/4 of the cheese, reserving the remaining 1/4 for topping the casserole.
Once the cheese is melted, stir in the green chilis.
In a skillet, brown the beef with the onions.
Add in the oregano, garlic powder and salt.
In a well greased 9 x 13-inch casserole dish, layer half of the tortilla strips.
Spread 1/2 of the beef mixture on top, followed by 1/2 of the cheese mixture; repeat layers; sprinkle reserved cheese on top.
Bake in pre-heated 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.
