An hour or two (or more is okay) before cooking, marinate/rub the tuna in the oyster sauce, and keep in refrigerator. Also, up to a day ahead, combine the grapefruit and orange juices in a sauce pan. Bring to a boil, and simmer until the juices are reduced by approximately half.

Before you start cooking the tuna, bring the grapefruit and orange juices to a simmer, and set aside. In a skillet, over a high heat, add the oil to the hot skillet, and then the tuna. Include any remaining oyster sauce marinade. Cook the tuna, on each side, to your liking (from rare to well done). If necessary, turn the heat down to prevent burning. If the pan seems to be a bit dry after a while, add two to three tablespoons of water to keep the fish from burning.

Remove the tuna from the pan and set aside. Turn up the heat under the tuna skillet. Add the heated juices to the tuna skillet and scrape up any tuna bits that may be left in the pan. Simmer the juices for a few minutes and season as needed with salt and pepper.

Pour the sauce over the tuna and serve with jasmine rice, or couscous.