Fill a kettle and boil it. Wash and peel the celeriac. Chop it into smallish pieces. Pop the celeriac in a pan. Pour in enough boiling water to cover it. Pop on a lid. Bring to the boil. Turn the heat down. Simmer for 15 mins till the celeriac is soft.
Trim the ends of the leek. Wash and finely slice it. Peel and finely chop the carrot. Rinse the celery and finely slice it. Rinse and slice the mushrooms.
Warm a drizzle of olive oil in another pan. Add the leek, carrot and celery. Gently fry for 5 mins till starting to soften. Stir every so often. Add the mushrooms. Fry for another 5 mins till the mushrooms are soft. Heat your oven to 180°C/Fan 160°C/Gas 4.
Take the veg off the heat. Strip the leaves from half the thyme sprigs. Stir into the veg with 3 tbsp of the tomato purée. Crumble in the stock cube. Drain and rinse the lentils. Add them to the pan with 150ml hot water from the kettle. Taste and season.
Drain the celeriac. Add the butter and some salt and pepper. Mash till smooth with a masher or fork. Coarsely grate the Cheddar and stir half of it into the mash.
Spoon the lentil mixture into an ovenproof dish or two individual ones. Top with the celeriac mash. Sprinkle over the rest of the Cheddar. Bake for 25-30 mins till golden and bubbling.
Trim the ends off the cavolo nero and shred it. Bring a pan of water to the boil. Pop the cavolo nero in a steamer basket or colander. Cover. Cook for 6-7 mins till it’s tender. Serve with the pie.