Preheat and prep: Preheat your oven to 425°F (220°C) and position a rack in the center. Put the carrots and parsnips on a baking sheet lined with parchment or a silicone mat. Drizzle 1½ tbsp olive oil over the roots and toss with ½ tsp kosher salt and ¼ tsp black pepper. Spreading them in a single layer ensures even contact with the pan so they caramelize instead of steaming.

Roast the roots: Roast the carrots and parsnips at 425°F for 20–25 minutes, turning once halfway through. Roast until edges are golden and a knife slips in easily—browning concentrates their natural sugars and creates the sweet-savory contrast that makes this salad special.

Bring steak to room temperature and season: While the roots roast, take the 540 g top sirloin out of the fridge for 20–30 minutes so it comes closer to room temperature (this helps it cook evenly). Pat it dry with paper towels and season both sides with the remaining 1 tsp kosher salt and ¾ tsp freshly ground black pepper. Dry surfaces brown better; salt now seasons the interior as the meat rests.

Make the vinaigrette: In a small bowl or jar whisk together the remaining 2½ tbsp olive oil, 2 tbsp balsamic vinegar (or 2 tbsp lemon juice), minced garlic, 1 tsp Dijon mustard, 1 tbsp chopped fresh herbs and a pinch of salt and pepper. Whisking (or shaking in a jar) emulsifies the oil and acid into a bright coating for the greens and veggies. Taste and adjust acid or salt—vinaigrette should be lively but balanced.

Sear the steak: Heat a heavy cast-iron skillet or grill pan over high heat until just smoking. Add a thin film of oil (a splash from the reserved oil). Place the steak in the hot pan—do not move it for the first 2–3 minutes to develop a deep brown crust (the Maillard reaction creates flavor). For a 540 g top sirloin (~1 to 1¼" thick), sear 3–4 minutes per side for medium-rare (internal ~125–130°F / 52–54°C). Sear the edges for 30–45 seconds to render fat. If you prefer medium, target 135°F (57°C).

Finish and rest the steak: Reduce heat to medium-low and add 1 tbsp unsalted butter and a sprig of thyme (if using) for the last 30–60 seconds, spooning the butter over the steak to add gloss and a hint of richness. Transfer the steak to a cutting board and tent loosely with foil; rest 8–10 minutes. Resting redistributes juices so they don’t run out when you slice.

Slice the steak: Using a sharp chef’s knife, slice the steak across the grain into 1/4"–1/3" slices. Cutting against the grain shortens muscle fibers, making each bite tender even with lean cuts like top sirloin.

Dress the greens and assemble: In a large bowl, place the mixed greens and dress lightly with about half the vinaigrette (reserve the rest for the vegetables or finishing). Toss gently—the goal is glossy leaves, not soggy ones. Arrange dressed greens on a serving platter or divide among 3 plates.

Warm the roasted roots and finish: When the roots are done, toss them briefly with the reserved vinaigrette (the warm veg will absorb flavor quickly) and scatter them over the dressed greens. Add slices of rested steak on top while still warm so juices mingle with the dressing.

Add avocado and herbs, finish to serve: Slice the 1½ avocados into wedges or dice and scatter across the plates. Sprinkle remaining chopped fresh herbs and a crack of black pepper. Serve immediately so the contrast between warm steak/roots and cool creamy avocado and crisp greens is at its best.