1. In a medium bowl, combine the flank steak with the water, cornstarch, oil, oyster sauce, light soy sauce, and baking soda. Marinate for 20 minutes.
2. Heat a wok over high heat until smoking.
3. Add 2 tablespoons of the oil, then add the flank steak in one layer.
4. Sear the flank steak undisturbed for 30 seconds, then stir-fry just until it turns opaque (it's okay if it's still slightly rare; it will be cooked again). Turn off the heat and transfer the beef to a plate, leaving behind any residual fat.
5. Reduce the heat to medium and add the remaining tablespoon oil, along with the onion and bell peppers. Stir-fry for 1 minute.
6. Stir in the chicken stock, curry powder, turmeric, salt, and sugar. Bring to a simmer.
7. Combine the cornstarch and water in a small bowl to make a slurry. Add about three-fourths of the slurry to the wok and simmer for 1 minute, until the sauce has thickened enough to coat a spoon. If the sauce isn't thick enough, add more slurry. If it's too thick, add more chicken stock. Check the seasoning and add salt if needed.
8. Stir in the cooked beef and cook for another 30 seconds. Serve.