Pat the beef sirloin dry with paper towels and season with salt and black pepper. If using, rub with minced garlic and let sit at room temperature for 15 minutes.
Heat a large skillet or wok over high heat. Add the vegetable oil and when hot, add the beef in a single layer (work in batches if necessary). Sear for 1-2 minutes per side until medium-rare.
Transfer the beef to a plate and let rest for at least 5 minutes to allow juices to redistribute.
While the beef rests, whisk together the lime juice, fish sauce, sugar, and chili flakes in a small bowl until the sugar dissolves.
In a large mixing bowl, combine the mixed greens, cucumber, carrot, red onion, cilantro, and mint leaves.
Slice the rested beef against the grain into thin strips.
Add the sliced beef to the bowl with the vegetables and herbs. Pour about three-quarters of the dressing over the salad and toss gently to coat.
Transfer to a serving platter or individual plates. Drizzle with remaining dressing and sprinkle with chopped peanuts. If using, add the sliced Thai chili and toasted rice powder.
Serve immediately with lime wedges on the side.
