Find a head of leafy celery. They're not that easy to find, as most producers tend to cut all the leaves off, but you're more likely to find one at a farmers' market. Remove all the leaves, keeping the rest of the celery for a soup, stock or salad. Roughly chop them, either in a food processor or by hand. Set your oven to its lowest temperature. With some modern ovens, you can just use the fan; and if you have an Aga, the warming oven is ideal. Scatter the celery on to 1 or 2 baking trays and leave in the oven overnight until the pieces are dry and crisp without being brown. Depending on how watery the celery is, you may have to allow even more time. Once they're dry enough, put them into a food processor with a handful of sea salt flakes and blend to a coarse powder-like consistency or finer if you wish. Store in airtight containers.