In a pot of boiling salted water, boil the orzo until it is al dente, drain it well in a colander.
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 then stir in the broth, the tomatoes, chopping them up with a spoon, the basil, and the cayenne.
Simmer the mixture, stirring, for 5 minutes, stir in the cooked orzo, 1/2 pound of the mozzarella and salt to taste, and transfer the mixture to a shallow 2-quart baking dish.
Add remaining mozzarella over the top of the orzo.
Bake the orzo in the middle of a preheated 400 degree oven for 30 minutes or until heated through and slightly browned and crisp on top.