Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking dish.
In a large skillet over medium-high heat, cook the ground beef and diced onion until the beef is browned and the onions are translucent. Stir in the minced garlic and cook for 1 additional minute. Drain any excess grease.
In a large bowl, whisk together the cream of mushroom soup, milk, Worcestershire sauce, salt, and pepper until smooth.
Stir the cooked beef mixture and the drained green beans into the soup mixture. Fold in 1/2 cup of the shredded cheddar cheese and 1/2 cup of the fried onions.
Pour the mixture into the prepared baking dish. Spread it out evenly.
Bake uncovered for 20-25 minutes, or until the casserole is bubbly and heated through.
Remove from the oven and sprinkle the remaining cheddar cheese and fried onions over the top.
Return to the oven and bake for another 5 minutes, or until the cheese is melted and the onions are golden brown. Let stand for 5 minutes before serving.