Season beef and sear in butter over high heat, 1–2 minutes per side in batches. Just build color.
Add sliced onion and beef to crockpot. Deglaze skillet with a splash of beef broth and pour into crockpot.
Add remaining broth, shoyu, brown sugar, garlic, ginger, sesame oil, vinegar and pepper. Stir.
Cook on LOW 4–5 hours or HIGH 2½–3½ hours. Check early for tenderness.
Remove 1 cup of sauce and bring to a boil in a saucepan. Mix cornstarch with cold water, whisk into boiling sauce until thickened, then return to crockpot.
Cook on HIGH 10–15 minutes until sauce is thick and glossy.
Loosen noodles under hot water. Add to crockpot with 1-inch green onion pieces. Toss and heat 5–10 minutes.
Finish with sesame seeds, green onions and chili crisp for heat!
Serve immediately! Don’t leave noodles on KEEP WARM or they’ll soak up the sauce.
