Place the oil and butter in a large, heavy skillet set over medium heat. Add the onion and cook, stirring occasionally, until the onions become tender. Add the garlic and cook until the garlic is fragrant but not browned. Remove the onions and garlic from the pan and set aside. Return the pan to the heat, and when it is hot, but not smoking, add the cod fillets and cook for about 1 minute.

Return the onions and garlic to the pan with the fish. Add the tomatoes, olives and thyme, sprinkling over and around the fish. Turn the heat to medium-low, cover the skillet with a tight-fitting lid and cook for about 3-4 minutes more, or until the fish is cooked and the onions are tender. Place a fillet on each plate, and spoon the onions, garlic and tomato mix over the fish, using all the juices that form in the pan. Sprinkle some fresh basil over each and serve immediately. 238 calories per serving; GL=1, Glycals=3