Preheat your oven to 375°F. Lightly grease a 9x13-inch baking dish with butter or spray. This helps prevent sticking and ensures your casserole cooks evenly from the edges to the center.

Heat olive oil in a large skillet over medium-high heat. Add ground beef and break it apart while cooking. Season with salt, pepper, garlic powder, and onion powder until fully browned.

Once the beef is fully cooked and no pink remains, carefully drain any excess fat using a colander. This step keeps the casserole from becoming too greasy and heavy.

In the same skillet, add diced onions and sliced mushrooms. Cook over medium heat, stirring often, until onions turn soft and mushrooms release moisture and become lightly golden and fragrant.

In a mixing bowl, whisk together cream of mushroom soup, milk, and Worcestershire sauce until smooth. This mixture creates a rich and flavorful base that ties the whole casserole together beautifully.

Add cooked beef back into the skillet with onions and mushrooms. Stir in green beans, then pour the creamy sauce over everything. Mix gently until all ingredients are evenly coated.

Pour the mixture into your prepared baking dish. Spread it out evenly using a spatula. Make sure the mixture reaches all corners so every serving has the same balance of ingredients.

Sprinkle shredded cheddar cheese evenly over the top of the casserole. This layer will melt during baking and create a rich, gooey texture that adds extra flavor to every bite.

Sprinkle one cup of French-fried onions over the cheese layer. This adds a crispy texture and delicious flavor that blends perfectly with the creamy filling underneath.

Cover the baking dish tightly with aluminum foil. Bake for 25 minutes to allow the casserole to heat through completely while keeping the moisture locked inside.

Remove the foil, sprinkle remaining onions on top, and bake uncovered for another 10 to 15 minutes until the top is golden, crispy, and the casserole is bubbling nicely.