In most French markets, you can buy just a single stalk of celery if that’s all you need for your broth. My relationship to celery changed a couple of decades ago when I first saw someone, French, bien sûr, peel a celery stock. When I asked her why, her answer was, “To make it more digestible.” I’m not certain that peeling the curved side of the celery — you do it with a vegetable peeler — makes it more digestible, but I do know that it’s a pleasure not to have to battle with stringy celery in polite company. These days, I routinely peel celery if I’m using good-sized hunks of it. If I’m thinly slicing it (and the strings would be short and not too problematic), I peel it only when I’m in the mood.