Preheat Oven:
Preheat oven to 350°F (175°C).

Cook Beef and Veggies:
In a large skillet, cook ground beef, onion, and bell pepper over medium heat until beef is browned and vegetables are tender. Drain excess fat.

Cook Macaroni:
Meanwhile, cook elbow macaroni according to package directions until al dente. Drain.

Mix Ingredients:
In a large bowl, combine cooked beef mixture, macaroni, diced tomatoes, cream of mushroom soup, milk, garlic powder, salt, and pepper. Stir well.

Assemble:
Transfer mixture to a greased 9×13-inch baking dish. Sprinkle shredded cheddar cheese evenly on top.

Bake:
Bake uncovered for 25–30 minutes or until cheese is melted and bubbly.

Serve:
Let cool for a few minutes before serving.