Place lemongrass, garlic, chilli, beef strips, fish sauce and corn-flour into a bowl, mixing well to combine. Allow to marinate while you prepare the remaining ingredients.
To make the pickles, combine radish wedges and cucumber slices in a bowl. Warm vinegar and caster sugar with a pinch of sea salt in a saucepan until dissolved, then pour over vegetables. Toss to coat; allow to marinate for at least 30 minutes. (Pickles taste better when left for longer, so it can be made in advance.)
Bring a medium-sized saucepan of water to the boil and cook bean thread noodles or rice vermicelli for 3 minutes. Drain, then run under cold water to separate. Use cooking scissors to cut into shorter strands.
Heat a large frying pan or wok over medium-high heat with the oil. Cook beef for 3-4 minutes or until golden brown. Add brown sugar and toss for another minute to coat.
To serve, arrange butter lettuce onto platter. Top with noodles, beef and pickles. Garnish with peanuts and herbs. Serve with a drizzle of Green Sriracha or chilli sauce if desired.