Place noodles in a large bowl and cover with hot water. Let stand for 4 minutes. Drain, run under cold water until noodles are cool and drain again. Using scissors, cut into manageable lengths and set aside.

To make dressing, combine fish sauce, lime juice, hot water and sugar in a bowl and stir until sugar has dissolved. Stir in chillies and set aside.

Set a cast-iron grilling pan over moderate heat for 5 minutes. Brush both sides of the steaks with oil. Cook for 4 minutes on first side, then for 3 minutes on second side for medium-rare. Remove to a warm plate, cover loosely with foil, and let stand for 10 minutes.

Meanwhile, spread watercress on a platter. Toss noodles with ¼ cup of the dressing and arrange over the watercress. Scatter over coriander and mint leaves. Combine onion and cucumber in a bowl and toss with 2 tablespoons of the dressing. When beef has rested, slice thinly and arrange over herbs and noodles, along with onion and cucumber. Drizzle with remaining dressing.