Preheat your grill to medium-high.
Combine the first 5 ingredients in a salad bowl and set aside. Brush the steak with the 1 tsp. oil and season with salt and pepper. Grill the steak to the desired doneness, allowing about 3 minutes per side for rare to medium rare. Remove the steak from the grill, cool 5 minutes, and then thinly slice.
Add the arugula to the dressing and toss to combine. Divide the arugula among 4 plates. Top the arugula with the sliced steak, potatoes and tomatoes, and then serve.