Preheat the oven to 350°F (175°C). Grease a 9×13-inch baking dish with a little butter or nonstick spray.
Bring a large pot of salted water to a boil and cook the egg noodles according to package instructions until al dente (usually 6–8 minutes). Drain and set aside. Toss the hot noodles with a splash of oil or a little butter to prevent sticking if desired.
While the noodles cook, heat a large skillet over medium heat. Add the ground beef and chopped onion. Cook, breaking the beef up with a spoon, until the meat is no longer pink and the onion is soft, about 6–8 minutes. Drain any excess fat from the skillet. Season the beef mixture with a pinch of salt and pepper.
In a large bowl, combine sour cream, cream of mushroom soup, and a pinch of salt and pepper. Stir until smooth and well blended. Taste and adjust seasoning if needed; the sauce should be slightly seasoned since the cheese will add salt.
In the prepared baking dish, layer half of the cooked noodles evenly across the bottom. Spoon half of the browned beef and onion mixture over the noodles and spread gently. Dollop and spread half of the sour cream mixture over the beef layer.
Repeat the layering: add the remaining noodles, top with the remaining beef mixture, and finish with the rest of the sour cream mixture spread evenly on top. Smooth gently with a spatula.
Sprinkle shredded cheddar cheese evenly over the top of the casserole.
Bake uncovered in the preheated oven for 25–30 minutes, or until the casserole is heated through, bubbling at the edges, and the cheese is melted and slightly golden. If you prefer a crispier top, switch to broil for 1–2 minutes—watch closely to avoid burning.
Remove from the oven and let the casserole rest for 5 minutes. Garnish with chopped parsley before serving.
Notes
For a healthier version, use lean ground beef or turkey, low-fat sour cream, and reduced-fat cheese. You can also add vegetables like spinach or peas for extra nutrients.