Mix cream of mushroom soup, mushrooms, butter, and onion in a slow cooker; add beef tips.
Cook for 8 hours on Low.
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.
Serve beef tips and sauce over cooked egg noodles.