Prepare vinaigrette: In a small bowl, whisk together salt, pepper, vinegar, half of the onion and the mustard until well combined.
Drizzle in the olive oil while still whisking. Taste and season with additional salt and pepper if desired; set aside. Heat a large non-stick skillet over medium-high heat until very hot. Pat steak dry with paper towels and season generously with salt and pepper; add to the skillet and brown for 2 to 4 minutes. Turn steak and cook for an additional 4 minutes or until desired level of doneness is achieved; transfer to a cutting board and allow steak to rest for 10 minutes.
Meanwhile, prepare salad: In a large bowl, combine salad greens, remaining onion, the tomatoes and half of the cheese. Add a little of the vinaigrette mixture and toss; add more and toss again until salad greens are well coated.
Arrange salad on dinner plates. Thinly slice steak against the grain (diagonally); serve over salads and garnish with remaining blue cheese.