Bring a large pot of lightly salted water to a boil. Cook egg noodles in the boiling water, stirring occasionally until cooked through but firm to the bite, about 5 minutes; drain.
Melt butter in a large skillet over medium-high heat. Saute onion in hot butter until translucent, about 5 minutes. Add ground beef to skillet in small chunks; cook and stir to break the beef into smaller pieces as it cooks until no longer pink, 5 to 7 minutes.
Sprinkle flour over the beef mixture, stir, and cook for 1 minute. Stir mushrooms and garlic salt into the beef mixture. Pour mushroom soup and chicken soup over beef, stir, and cook until hot, about 5 minutes; add sour cream, stir until smooth, and cook until again hot, 2 to 3 minutes more. Pour beef mixture over the cooked egg noodles.
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