Rub tuna steaks with peppercorns and set aside while you make the salsa.
To make the salsa, section oranges, removing all peels and pith. Chop orange sections into 1/2-inch pieces. Combine orange with onion, cilantro, olive oil and lime juice. Toss together, season with salt and cayenne and set aside. Rub a cast iron grill pan with oil and heat. Grill tuna, turning once, until desired degree of rareness is reached, from 2 to 5 minutes per side, depending on thickness. Serve topped with citrus salsa.