Freeze the beef tenderloin for about 2 hours, then slice it thin against the grain.
Mix the marinade: dipping powder, light and dark soy, oyster sauce, five-spice, baking soda, white pepper, and oils.
Season generously and pour in extra neutral oil.
Mix well and marinate for 30 to 40 minutes.
Slice the green pepper, onion, ginger, garlic, and chilies while the beef marinates.
Heat the oil to about 80 percent hot, then fry the ginger, garlic, and chilies until fragrant.
Add the marinated beef and toss quickly over the highest heat.
Stir-fry about 1 minute until the beef changes color.
Add the green pepper and onion and toss until bright and crisp.
Squeeze in lemon, stir-fry 3 to 4 minutes total at most, confirm the beef is cooked through, and season with salt at the very end.
Plate and garnish with scallion greens and roasted sesame.
Serve immediately while the wok flavor is fresh.