Fill a large pan with water. Sprinkle in a little salt, cover and bring to the boil. Chop the roots off the celeriac and peel off the outer skin. Chop into small, bite-sized chunks. When the water is boiling, slide in the celeriac. Simmer for 20 mins, till tender when crushed with a fork.
Meanwhile, peel the onion and finely chop it. Peel the garlic cloves and crush or finely chop them. Peel the carrot and finely chop it. Chop the tomatoes into small pieces. Pick the thyme leaves off their sprigs.
Pour 1 tbsp oil into a pan and bring to a medium heat. Slide in the chopped onion, carrot and garlic. Sprinkle in the thyme leaves and a pinch of salt and pepper. Fry for 5-6 mins, stirring occasionally.
Stir the soya mince into the vegetables. Drain and rinse the lentils. Shake dry and tip into the pan. Slide in the tomatoes. Crumble in the stock cube. Pour in 100ml boiling water and 2 tbsp Worcester sauce. Stir well to combine, and bring to the boil. Reduce the temperature to a simmer and cook for 10 mins, stirring occasionally.
When the celeriac is tender, drain it and give it a good mash.
Preheat your grill to medium-high. Spoon the cooked mince into an ovenproof casserole dish and flatten to an even layer. Scoop a layer of celeriac mash on top.
Slide the pie under the grill. Cook for 6-8 mins, till bubbling and a little coloured on top. Serve straight away.