Prepare the noodles: Bring a large pot of water to a rolling boil. Add the rice vermicelli and cook according to the package instructions — usually 3 to 4 minutes. Stir them gently with a pasta fork; you'll know they're ready when they turn from opaque white to a translucent, glossy finish and feel tender but still offer a slight resistance when bitten. Drain immediately into a colander and rinse under cold running water until the noodles feel cool to the touch. Shake off the excess water and set aside.

Make the dressing: In a small bowl, whisk together the fish sauce, lime juice, brown sugar, minced garlic, and half the chopped chilli (if using). Stir until the sugar has completely dissolved — you should see a clear, amber-coloured liquid that smells pungent and citrusy all at once. Set aside.

Marinate the beef: Place the thinly sliced beef in a medium bowl. Pour over half of the dressing and add the chopped lemongrass. Toss everything together with your hands until each piece of beef is evenly coated. Let it sit for 10 minutes at room temperature — the meat will take on a darker, bronzed appearance as it absorbs the marinade.

Cook the beef: Heat a large frying pan over high heat until it's smoking hot — you should see a faint wisp of smoke rising. Add the vegetable oil and swirl to coat the surface. Carefully lay the beef slices in a single layer, working in batches if necessary to avoid overcrowding. You'll hear a satisfying sizzle as the meat hits the pan. Cook for 1 to 2 minutes without moving them — the edges will turn brown and crisp. Flip each piece with tongs; the cooked side should be a rich mahogany colour with charred edges. Cook for another 30 to 60 seconds for medium, then transfer to a plate. The beef should be tender with a slight chew and smell deeply savoury and slightly caramelised.

Assemble the bowls: Divide the cold noodles among four serving bowls. Arrange the julienned carrot, cucumber matchsticks, beansprouts, mint leaves, and coriander leaves on top in neat piles. Add the warm beef slices and scatter over the chopped peanuts and remaining chilli.

Finish and serve: Drizzle the remaining dressing over each bowl just before serving. Toss everything together at the table so the noodles catch every bit of the tangy, savoury dressing. The first bite should be a symphony of cool, crunchy, warm, and fragrant all at once.