1. Put the tuna in a nonmetallic shallow dish. Whisk together 4 tbsp oil, 1 garlic clove, 1 tbsp lime juice, and the rosemary in a small bowl; season with pepper. Pour over the tuna, making sure both steaks are coated. Cover with plastic wrap and marinate in the refrigerator for 1 hour.

2. Meanwhile, to make the salsa, put the tomatoes, scallions, and hot pepper in a nonmetallic bowl. Stir in 2 tbsp olive oil and the remaining garlic and lime juice, then season with salt and pepper. Cover and refrigerate until ready to serve.

3. Ten minutes before grilling, remove the tuna and salsa from the refrigerator. Heat the ridged grill pan over high heat until a splash of water "dances" on the surface. Brush the ridges with a little of the marinade.

4. Remove the tuna from the marinade and reserve the marinade. Place the tuna on the grill pan and cook for 3 minutes, brushing with the marinade. Carefully turn over the tuna and grill for another 2–4 minutes, until the outside is browned but the meat is still pink in the center.

5. Transfer to a cutting board and let stand for 2 minutes. Cut each steak in half crosswise. Combine the arugula and salsa and serve with the tuna, garnished with lime wedges.