In a kettle of boiling salted water boil the orzo until it is al dente, drain it well, and transfer it to a large bowl. In a large heavy skillet cook the onion in the oil and the butter over moderately low heat, stirring, until it is softened, add the celery, and cook the mixture, stirring, for 5 minutes. Stir in the flour, cook the mixture, stirring, for 3 minutes, and stir in the broth, the tomatoes, chopping them with a spoon, the basil, and the cayenne. Simmer the mixture, stirring, for 5 minutes, stir it into the orzo with the olives, 1/2 pound of the mozzarella, cut into 1/4-inch dice, and salt to taste, and transfer the mixture to a shallow 2-quart baking dish. Cut the remaining 1/4 pound mozzarella into thin strips and arrange it decoratively on top of the orzo. The orzo may be prepared up to this point 1 day in advance and kept covered and chilled. Bake the orzo in the middle of a preheated 400F. oven for 30 minutes, or until it is heated through and slightly crisp on top.

Serves 12.