Preheat the oven to 325°F (165°C).

Grease a 9x13-inch baking dish.

In a skillet, heat the oil over medium heat.

Add the diced onion and ground round beef.

Cook, stirring occasionally, until the beef is browned and the onion is soft.

Drain excess fat if needed.

In a saucepan, combine the cream of chicken soup and milk.

Cook over medium heat, stirring constantly, until smooth.

Stir in the diced green chiles.

Cut the corn tortillas into 1-inch squares.

Arrange half of the tortilla squares in a single layer on the bottom of the prepared baking dish.

Spread half of the cooked ground beef mixture over the tortilla layer, then pour half of the soup mixture evenly over the beef layer.

Sprinkle half of the shredded cheddar cheese on top.

Repeat the layers with the remaining tortilla squares, beef mixture, soup mixture, and shredded cheddar cheese.

Bake in the preheated oven for 20 to 30 minutes, or until the casserole is bubbly and the cheese is melted and golden brown on top.

Remove from the oven and let it cool for a few minutes before serving.

Serve hot and enjoy this cheesy and satisfying beef tortilla casserole!