Rub 1 tablespoon of the oil over the steaks and season them with salt and freshly ground black pepper. Cook them in a hot griddle pan for 3 minutes on each side, then set them aside to rest for 3–4 minutes. Slice into thin strips.
Heat the rest of the oil in a wok over a high heat until the oil starts to shimmer, then add the onion. After 3 minutes, when it starts to soften, add the peppers, lemon grass, chillies, garlic and ginger. Then add the oyster sauce and measurement water and bring to the boil.
Remove from the heat and fold in the basil and reserved steak strips. Serve immediately.