Bring the beef to room temperature (15–20 minutes). Pat the 200 g sirloin dry with paper towels and season lightly with salt and several grinds of black pepper. Bringing the meat to room temperature ensures even cooking; patting it dry and seasoning helps the surface brown quickly and develop a flavorful crust (Maillard reaction).

Make the dressing: in a small bowl whisk together the juice of the lime (about 2 tablespoons), 1 tsp fish sauce, 1 tbsp olive oil, the minced garlic (if using) and 1 tsp brown sugar (optional). Taste and adjust — the goal is a bright balance of salty (fish sauce), sour (lime), fat (olive oil) and a hint of sweetness. Whisking emulsifies the oil into the lime juice so the dressing coats everything evenly.

Prep the produce: thinly slice the cucumber on the diagonal into 2–3 mm slices (or use a peeler to make ribbons if you prefer texture contrast). Halve the cherry tomatoes. Tear the mint leaves and roughly chop the cilantro. Toss the 3 cups mixed greens, cucumber and tomatoes together in a large bowl and season lightly with a pinch of salt — salting the vegetables early brings out their natural flavors.

Toast the cashews: heat a small dry skillet over medium heat and add the 1 tbsp cashews. Shake the pan or stir frequently for 2–3 minutes until fragrant and just golden. Toasting intensifies the nutty aroma and adds crunch; watch closely to avoid burning. Remove from the pan and roughly chop once cool.

Sear the beef: place a heavy skillet (cast-iron preferred) over high heat and add 1 tsp neutral oil. When the oil is shimmering and just starting to smoke, add the sirloin. Sear without moving for 2–3 minutes to get a deep brown crust, then flip and sear the other side 2–3 minutes for medium-rare (time will vary by thickness). High-heat searing creates a savory crust while keeping the interior juicy. If you prefer medium, add 1 more minute per side.

Rest and slice: transfer the seared sirloin to a cutting board and let it rest 5–7 minutes. Resting lets juices redistribute so the meat stays moist when sliced. Then slice thinly against the grain into 3–5 mm slices — cutting against the grain shortens muscle fibers and makes each bite tender.

Dress the salad: toss the greens, cucumber and tomatoes with about two-thirds of the dressing (reserve the rest). Toss gently so leaves aren’t bruised; the reserved dressing can be used later on the beef if desired. Arrange the dressed salad on a serving platter or divide between two plates.

Assemble and finish: fan the sliced beef over the salad, scatter torn mint, chopped cilantro and the toasted cashews. Drizzle the remaining dressing over the beef and finish with an extra squeeze of lime and a final crack of black pepper. The contrast of warm beef and cool, herb-flecked salad is key to the classic Thai yin–yang of textures and temperatures.

Serve immediately. This salad is best enjoyed right away so the greens stay crisp and the beef is still slightly warm, which brightens the dressing and releases the aromatics of the herbs.