Sprinkle tuna steaks with 1 teaspoon pepper and 1/4 teaspoon salt. Heat 1 tablespoon oil in a large nonstick skillet over medium-high heat. Add tuna steaks; cook 2 minutes on each side or until desired degree of doneness.
Combine 1/2 teaspoon pepper, 1/2 teaspoon salt, 1 tablespoon oil, and juice in a large bowl; stir with a whisk. Add arugula and fennel; toss well. Place about 2 cups salad on each of 4 plates; top each serving with 1 tuna steak.