Season steak with 1 tsp salt and 1/2 tsp pepper. Heat olive oil in a cast-iron skillet over high heat until smoking. Sear steak 4-5 minutes per side for medium-rare (130°F internal). Transfer to a cutting board, tent with foil, and rest 5 minutes.
Meanwhile, make dressing: In a bowl, whisk together mayonnaise, sour cream, buttermilk, 2 oz crumbled blue cheese, vinegar, garlic powder, and a pinch of salt and pepper until smooth. Refrigerate until ready to use.
In a large bowl, combine salad greens, tomatoes, red onion, and 1/2 cup blue cheese crumbles. Toss with about 1/2 cup of dressing (or to taste).
Slice steak thinly against the grain. Divide salad among plates, top with steak slices, and drizzle with remaining dressing if desired. Serve immediately.
Tips
For best results, let steak come to room temperature 20 minutes before cooking.
Dressing can be made up to 3 days ahead and stored in the fridge.
Substitute steak with grilled chicken or shrimp if desired.