As the days grow cooler, Lone Acre Farm is shedding the last of its first summer's harvest and sprouting cold-weather crops aplenty. Celeriac, or celery root, is one of the newcomers, and Machin's young, fresh bulbs are as good as they get. It's hard to imagine that this root vegetable was almost unheard of until a few years ago, when chefs embraced it and its popularity began to spread. You'll have no trouble spotting celeriac at markets this time of year, thanks to its seasonality as well as its distinctively knobby appearance. Once peeled, roasted, and made into a hearty vegetable stew, or pureed into a creamy celery root potatoes, it's a savory case in point that some vegetables are worth discovering.