Maybe Claire, knowing her husband's appetite, had prepared sandwiches in advance and kept them in the refrigerator. In a Tupperware container, of course.
And once again you may have hit on something - I can definitely see that as a possibility. I mean, given the way things play out so quickly in the script with regard to Claire preparing the sandwiches and getting the coffee going, she would have otherwise have to have been a genius at putting together sandwiches so amazingly quickly.
And speaking of Tupperware, something just hit me: If Claire and Alex really were as broke as Alex claimed on the night they had dinner at Collinwood with Tracy and Quentin, the perfect solution could have been that Tracy and Quentin wouldn't have abandoned the tradition of serving tea in the drawing room and those continued tea events could have been combined with Tupperware parties hosted by Claire to make extra money!
It would have been a win-win. But alas, no one lived long enough for them to do it. Oh well. But can you just imagine the scene, though - especially how Carlotta would have most likely looked on disapprovingly!!