LENOX - If you see something you like on the menu at Nudel, a cozy downtown restaurant here, order it now. The dish may not be there tomorrow. But it’s easy for Nudel’s fans, and anyone curious about chef-proprietor Bjorn Somlo’s penchant for culinary invention, to keep up: the evening’s menu is posted online around 5 p.m. each day (the restaurant tweets updates on the go). Nudel turns the neat trick of garnering loyal patrons while constantly changing offerings. Its menu reflects both the whimsy and restlessness of a young chef-auteur, and the vagaries of seasonality and the irregular yield of small farms.