Season the steak. Pat the steak dry with paper towels.
Season both sides with salt and pepper. Let it sit at room temperature for 20–30 minutes to relax the fibers and promote even cooking.
Toast the nuts. In a dry skillet over medium heat, toast walnuts or pecans for 3–4 minutes, stirring often. When fragrant and lightly browned, transfer to a plate to cool.
Make the dressing. In a small bowl or jar, whisk olive oil, vinegar, Dijon, honey, garlic, salt, and pepper until emulsified.
Taste and adjust: add a pinch more salt or a splash of vinegar if it needs brightness.
Prep the salad base. In a large bowl, combine romaine, arugula, tomatoes, red onion, cucumber, and avocado. Don’t toss with dressing yet.
Sear the steak. Heat a heavy skillet (cast iron is best) over medium-high until hot. Add olive oil.
Lay in the steak and sear without moving for 2–3 minutes, until a deep crust forms. Flip, add butter, smashed garlic, and herbs. Tilt the pan and baste the steak with the melted butter for another 2–4 minutes, depending on thickness and preferred doneness.
Check doneness. For medium-rare, aim for an internal temp of 130–135°F (54–57°C).
Remove to a cutting board and rest for 5–10 minutes so the juices redistribute.
Slice the steak. Slice thinly against the grain. This keeps the bite tender and pleasant.
Dress and assemble. Lightly dress the greens with half the dressing and toss to coat. Plate the salad, top with sliced steak, crumbled blue cheese, and toasted nuts.
Drizzle with a little more dressing and finish with freshly cracked pepper. Add optional croutons or balsamic glaze if you like.
Serve immediately. This salad shines when the steak is still warm and the greens are crisp.