To make the dressing: In a medium bowl, whisk together the buttermilk, mayonnaise, garlic, lemon juice and a pinch of salt. Stir in the blue cheese. Add more salt and pepper to taste. Chill in the refrigerator for up to 12 hours.
Preheat oven to 400 degrees F. Salt and pepper both sides of the steak.
In a heavy, oven-safe frying pan over medium-high, heat the canola oil. Brown the steak for 5 minutes on each side. Transfer the pan to the oven and cook 20 minutes for medium-rare, depending on the steak’s thickness. Transfer the steak to a cutting board, let it rest for 10 minutes and then thinly slice the steak.
In a large bowl, toss the arugula with the olive oil, lemon juice and 1/4 teaspoon salt.
Divide the arugula among 4 plates. Place 3 or 4 slices of steak on each plate and sprinkle the tomato halves over the top. Drizzle with blue cheese dressing and serve.