Mix the salt and celeriac, put into a plastic container, and cover. Leave in the fridge for 2 days, allowing time for the celeriac and salt to get to know each other. Then lay out the mixture in a baking pan and bake in a 200°F oven for about 2–3 hours until thoroughly dried out and crisp; check frequently to avoid any singeing as this will give the celery salt a burnt taste. Crush the dried salt and celeriac in a food processor or use a mortar and pestle. The celery salt will keep for ages in an airtight container.