Cover the slow cooker with the lid and cook the beef and soup mixture on HIGH for 2 to 2 1/2 hours, or on LOW for about 4 hours, until the ground beef is cooked through and the mixture is hot and bubbly around the edges.
Once the beef is cooked, give the mixture a good stir, breaking up any larger pieces of meat and mixing the sauce and beef together. Taste a small bit and add salt and pepper if needed, keeping in mind the soup and cheese will both add salt.
Stir in the uncooked egg noodles, pressing them down gently so they are mostly submerged in the creamy beef mixture. Try to keep the noodles in an even layer so they cook at the same rate.
Re-cover the slow cooker and cook on HIGH for 20 to 30 minutes, or until the noodles are just tender but not mushy. Lift the lid once or twice to gently stir and press any dry noodles down into the liquid so they soften evenly.
When the noodles are tender, turn the slow cooker to LOW or the KEEP WARM setting. Gently stir in the sour cream until the sauce is smooth and creamy, then sprinkle the shredded cheddar cheese evenly over the top.
Cover and let the cheese melt for about 5 to 10 minutes. Once the cheese is melted and the casserole is creamy and thick, give it a gentle stir if you like a more blended casserole, or leave the cheesy layer on top for a more traditional look.
Serve the Yumasetti hot, straight from the slow cooker, making sure each serving has a good mix of beef, noodles, and sauce. Leftovers keep well in the refrigerator for up to 3 days and can be reheated gently with a splash of broth or milk if they seem too thick.
