Toss the strips of steak, the lemongrass, soy sauce, half the fish sauce, half the sugar and half the chilli flakes together in a bowl. Set aside in the fridge to marinate for at least 20 mins or up to 6 hrs.
Mix the remaining fish sauce with the lime juice, remaining chilli flakes, the rest of the sugar and 3 tbsp water, then set aside.
Heat 1 tbsp vegetable oil in a large non-stick or cast iron wok or frying pan over a high heat. Once hot, tip in half the marinated steak strips and stir-fry for 2 mins until just cooked through, then remove to a large plate using tongs or a slotted spoon and repeat using another 1 tbsp of oil and the remaining beef. Remove to the plate, then wipe the wok or pan clean using kitchen paper.
Add the remaining oil to the pan and fry the green pepper and the white parts of the spring onion. Stir-fry over a high heat for 2-3 mins, then add the garlic and ginger and stir-fry for another 45 seconds-1 min. Return the steak to the pan, stir well, then add the lime dressing and most of the basil leaves, mixing well to coat.
Divide the stir-fry between bowls alongside rice or noodles, then scatter with the chopped peanuts, the remaining basil leaves and the green parts of the spring onions.
