In a skillet, cook the ground beef and chopped onion over medium heat until the beef is browned and the onion is tender. Drain any excess fat.
In a large bowl, combine the cooked beef and onion, mixed vegetables, cream of mushroom soup, shredded cheddar cheese, milk, salt, and black pepper. Mix well.
Transfer the mixture to a baking dish and spread it out evenly.
Bake in the preheated oven for 45 minutes, or until the casserole is bubbly and the top is golden brown.
Let the casserole cool for a few minutes before serving.
Notes:
Feel free to add more vegetables or use a different type of cheese to suit your taste.