2Heat a skillet over high heat with the butter and sear the beef in small batches for 1-2 minutes per side to build color.
3Transfer the beef to the crockpot. Deglaze the skillet with a splash of the measured beef broth, scrape up the browned bits, and pour everything into the crockpot.
4Add the remaining broth, shoyu, brown sugar, garlic, ginger, sesame oil, and black pepper. Stir gently.
5Cover and cook on LOW for 4 to 5 hours or HIGH for 2.5 to 3.5 hours, checking early to keep the thin beef tender.
6Mix the cornstarch slurry with a cup of broth. Stir the slurry into the sauce and cook on HIGH for 15–20 minutes, until thick and glossy.
7Slice the green onions into 1 inch pieces. Separate the precooked noodles and add them directly to the crockpot. Toss everything together and heat on HIGH for 5–10 minutes.
8Finish with sliced green onions, sesame seeds, and chili crisp oil if heat is desired, then serve immediately.