Cook green pepper and onion in oil until tender. Blend in flour; add milk and cook, stirring constantly, until thickened. Add the salt and mustard. Remove from heat. Add cheese and stir until it is melted. If necessary, place pan over very low heat to melt cheese. Stir sauce into eggs; add rice. Turn into greased custard cups. Bake at 350 degrees for 35 minutes or until firm. Unmold and serve plain or with tomato sauce.