Many of the jokes are constructed as twists on the Sorting Hat's "House X" pattern, and common food items prepared at home are called "House X". For instance "House Pickled Strawberries" prepared in a jar with a sweet brine. "Stewberries" is a corruption of "strawberries". (There's one blog out there suggesting it may have something to do with Jon Stewart of the Daily Show, but I think it more likely that it's meant to be the way a very young child would pronounce "strawberries".)