Prepare the Beef
If needed, freeze the beef for 30 minutes to make slicing easier.
Slice beef into thin strips against the grain.
In a bowl, combine beef with sesame oil, cornstarch, baking soda, and salt. Massage well and let marinate for 15 to 30 minutes.
Prepare the Sauce
In a small bowl, whisk together hoisin sauce, soy sauce, rice vinegar, and brown sugar.
In another bowl, mix cornstarch and water to create a slurry.
Mince garlic and ginger. Slice bell peppers, onions, and green onions.
Stir Fry
Heat one-third of the oil in a large skillet or wok over medium-high heat.
Add beef in a single layer and cook for 2 to 3 minutes per side until browned. Cook in batches if necessary. Remove and set aside.
Add another third of the oil and stir-fry bell peppers and onions for 2 to 3 minutes until tender-crisp. Remove if desired.
Add remaining oil, reduce heat to medium, and sauté garlic and ginger for 1 to 2 minutes until fragrant.
Pour in the stir-fry sauce and cornstarch slurry, stirring constantly until glossy and thickened.
Return beef and vegetables to the pan along with green onions. Toss everything together until coated.
Garnish with sesame seeds and serve immediately.