In a bowl, combine the minced lemongrass, garlic, fish sauce, and thinly sliced beef sirloin. Let marinate for 15 minutes.

Cook the rice vermicelli noodles according to package directions, then rinse under cold water and drain well.

Heat a large skillet over medium-high heat. Add the marinated beef and sear for 2-3 minutes per side until fully cooked.

Divide the chilled noodles among four bowls. Top with the cooked beef, julienned cucumber, shredded carrots, mint, and cilantro.

Drizzle with a simple dressing (like lime juice, water, a pinch of sugar, and an extra splash of fish sauce) before serving immediately.