STIR arugula with garlic, lemon zest and juice, and 1/4 tsp salt in a medium bowl.
SPRINKLE steak with 1/4 tsp salt. Season with fresh pepper. Heat an extra-large non-stick frying pan over medium-high. Add 2 tsp oil, then steak. Reduce heat to medium. Cook 6 to 7 min per side for medium-rare. Transfer to a plate and tent with foil.
HEAT remaining 4 tsp oil in same pan over medium-high. Add potatoes and cook, stirring often, until fork-tender, 15 to 18 min. Add onion and cook until golden, 2 to 3 min. Stir in tomatoes and remaining 1/4 tsp salt. Season with fresh pepper.
SLICE steak thinly, then divide among 4 plates. Top with arugula mixture.