I seem to have invented a recipe... (Celeriac is my latest culinary discovery. It tastes very like celery but it's a root vegetable so the texture is entirely different) Leek, Potato And Celeriac Pie Make a white sauce- add a generous teaspoon of whole grain mustard. Cut a large potato, a large leek and a sizeable piece of celeriac into small chunks and simmer in the sauce until soft. Season with salt and pepper and add a generous scattering of coriander seed. Transfer mixture to pie dish and cover with pastry. Cook until pastry is done.