1. Whisk together lemon zest, apple juice, lemon juice, mint, mustard, salt and pepper in a small bowl. Place steak in a shallow bowl and coat with 3 tablespoons dressing. Flip, cover and set aside. Add oil to remaining dressing.
2. Preheat the broiler. Place potatoes in a small saucepan with enough water to cover. Bring to a boil over high heat, reduce heat to low, cover and simmer until tender, 15 to 20 minutes. Drain, cool and cut into 1/2 -inch-thick slices.
3. Meanwhile, broil steak 4 inches from heat for 8 minutes for medium rare, 10 minutes for medium and 12 minutes for well-done, flipping halfway through. Remove from oven and let rest 5 minutes. Cut into 1/4 -inch-thick slices.
4. Divide arugula, radicchio, fennel and tomatoes among four plates. Top with potatoes and steak; drizzle with steak juices and dressing.