Heat the oil in a large saucepan and add the onions. Cook for about 2 minutes before adding the garlic, leeks, carrots and celery. Saute for about 5 minutes.
Add in the tinned tomatoes, passata and dried herbs and bring to the boil, then turn down the heat and leave to simmer for 30 minutes.
Meanwhile, gently boil the sweet potato with the celeriac for about 25 minutes (until a fork goes into them easily). Drain them and put them back in the saucepan, add a knob of butter and mash with a fork or potato masher. You can add some milk or water if the mash is too dry.
Once the veg and tomatoes have been simmering, add in the lentils, beans and parsley. Stir in the worcestershire sauce, adding a little at a time to taste. Season with salt and pepper and place the veg filling in the bottom of a large ovenproof dish. Cover with the mash and some grated cheese if using.
Bake the pie in a hot oven for 20 minutes or until bubbling at the edges. We love to serve this pie with some steamed or sautéed greens - spinach, tender stem broccoli or chard.