Place an upturned saucer or metal pastry ring in the base of the ceramic cooking pot.
Pour in about 2.5cm/1in hot water and switch on the slow cooker
Lightly grease the base and sides of a 900g/2lb loaf pan or terrine - first making sure it will fit in the slow cooker - and line the base and sides of the tin with baking parchment
Heat the oil in a large pan, add the celery, mushrooms and garlic, then cook for 10 minutes, until the vegetables have softened, do not let them brown.
Remove the pan from the heat, then stir in the lentils, mixed nuts and flour, grated cheese, beaten egg and herbs. Season well with salt and black pepper and mix thoroughly.
Spoon the nut mixture into the prepared loaf tin or terrine, pressing right into the corners. Level the surface with a fork, then cover the tin with a piece of foil. Place the loaf tin in the ceramic cooking pot and pour in enough near-boiling water to come just over halfway up the side of the dish.
Cover the slow cooker with the lid and cook for 3-4 hours or until the loaf is firm to the touch.
Leave the loaf to cool in the tin for about 15 minutes, then turn out on to a serving plate. Serve the loaf hot or cold cut into thick slices and garnished with fresh chives and sprigs of flat leaf parsley.