Preheat your oven to 350°F (175°C).
In a large skillet, brown the ground beef over medium heat until fully cooked. Drain excess fat.
Add the diced onion to the skillet and sauté until translucent.
Stir in the egg noodles, cream of mushroom soup, sour cream, frozen mixed vegetables, garlic powder, salt, and pepper.
Transfer the mixture to a greased 9x13 inch baking dish.
Sprinkle shredded cheddar cheese evenly on top.
Cover with foil and bake for 30 minutes. Remove foil and bake for an additional 15 minutes until the cheese is bubbly and golden.
Let the casserole sit for 5-10 minutes before serving.