Prepare Beef
Freeze beef 15 minutes. Slice thinly across the grain.
Combine baking soda and 2 tablespoons water. Toss with beef. Let sit 5 minutes.
Cook Noodles
Cook noodles in salted water until al dente. Drain and rinse.
Make Sauce
Combine soy sauce, hoisin, garlic, ginger, and red pepper flakes.
Cook Vegetables and Beef
Heat oil. Cook beef until crisp outside, pink inside (about 3 minutes). Stir in 1 tablespoon sauce, cook 30 seconds. Remove beef.
Add carrots, broccoli, and bell pepper. Cook 2 minutes.
Add remaining 4 tablespoons water. Steam until bright green and liquid is mostly gone (about 2 minutes).
Stir in water chestnuts, half the green onions, and 2 tablespoons sauce. Cook 30 seconds.
Combine and Finish
Add noodles and beef. Pour remaining sauce over.
Toss until noodles are heated through.
Drizzle with sesame oil (if using) and sprinkle with remaining green onions. Toss.
