Bring a large saucepan of salted water to a boil for orzo.
In a heavy skillet heat oil and butter over moderate heat until hot but not smoking and cook fennel, onion, tomatoes, and tarragon until fennel is tender, about 5 minutes. Season mixture with salt and pepper and keep warm, covered.
Cook orzo in boiling water until al dente and drain in a colander. In a bowl stir together orzo, fennel mixture, and salt and pepper to taste.
