Preheat oven to 350°F (175°C).
In a skillet, cook ground beef and onion until the beef is browned; drain excess fat.
Stir in cream of mushroom soup, diced tomatoes, water, soy sauce, garlic powder, and pepper.
Let the mixture simmer for 5 minutes to blend flavors.
Grease a 2-quart casserole dish and layer half of the chow mein noodles on the bottom.
Spread half of the beef mixture over the noodles.
Repeat layers with the remaining noodles and beef mixture.
Bake uncovered for 25–30 minutes, until heated through and slightly crispy on top.
Let cool slightly before serving.