Cut the turnip, potato, onion, celeriac and carrot into uniform cubes about 2.5 cm / 1 inch across. Cut the garlic cloves into paper-thin slices. Keep the component elements separate.
Put a large, heavy pan over a low heat and pour in all the oil. Sweat the carrot and onion in this for 10 minutes, stirring occasionally, then add the rest of the vegetables except the leeks. Stew together