Freeze the steak for about 20 minutes to firm it, then slice against the grain into ½ cm (thin) slices; season the strips with salt and black pepper.
Heat 2 tablespoons oil in a large wok or frying pan over high heat until very hot to ensure a good sear.
Working in one or two batches to avoid crowding, fry the beef undisturbed for 4 minutes, then turn and cook 1–2 more minutes until just cooked through; transfer to a plate and set aside.
In the same pan, add the minced ginger, garlic and red pepper flakes and sauté for about 1 minute until fragrant.
Add the tamari (or soy sauce/coconut aminos), brown erythritol and water; turn the heat up to bring to a boil, then reduce to medium and simmer 3–4 minutes until the sauce reduces and thickens slightly.
Return the cooked beef to the pan along with the sliced red bell pepper and cook 2–3 minutes more until peppers are just tender but still crisp.
Turn off the heat, stir in the sliced spring onions, sprinkle with toasted sesame seeds and drizzle with toasted sesame oil if using; season with extra black pepper to taste and serve.
