Preheat your oven to 350°F (175°C).
In a large skillet over medium heat, brown the ground beef along with the diced onion until the beef is no longer pink and the onion is translucent, about 5-7 minutes. Drain excess fat if necessary.
In a pot, bring water to a boil and cook the egg noodles according to package instructions until al dente. Drain and set aside.
In a mixing bowl, combine the cream of mushroom soup, sour cream, garlic powder, salt, and black pepper. Mix until well combined.
Add the cooked ground beef and onions to the mixing bowl with the creamy mixture, followed by the cooked egg noodles. Stir until everything is evenly coated.
Transfer the mixture into the greased casserole dish, spreading it evenly.
Sprinkle the shredded cheddar cheese on top of the casserole.
Cover the casserole with aluminum foil and bake in the preheated oven for 30 minutes.
Remove the foil and bake for an additional 15 minutes, or until the cheese is melted and bubbly.
Allow the casserole to cool for a few minutes before serving.