Peel the celeriac and place on a large piece of aluminium foil. Sprinkle with salt and top with butter. Wrap it in foil and bake for approx. 1½ hours at 150°C or until very tender. Allow to cool slightly, then cut into smaller pieces.Sugar-salted leeks
Combine the sliced leeks with sugar and place in a sieve. Let them drain for at least 30 minutes.Buttermilk dressing
Beat the buttermilk, cream, lemon zest and juice, salt and freshly ground black pepper until creamy. Season to taste.
Combine the celeriac, leeks, quinoa and apples. Arrange in a dish and pour the buttermilk dressing over everything.