Peel and segment the oranges. Whisk together the orange juice, wine (if using) and cornflour in a small saucepan until smooth. Bring the mixture to the boil over a medium-high heat then cook, stirring, for about 2 minutes or until the sauce thickens. Remove from the heat and stir in the orange segments. Keep warm.
Mix the coriander, cornmeal and pepper in a shallow dish. Coat both sides of the tuna steaks with the cornmeal mixture, pressing firmly so it adheres.
Heat 2 teaspoons of the oil in a large, heavy frying pan over a medium-high heat until hot but not smoking. Sear the tuna for about 2–3 minutes on each side for medium-rare or longer according to your taste. Add the remaining oil just before turning the fish. Serve with the sauce.