Peel and dice your celeriac. If it's covered in mud you might find it useful to do this in the sink. Use a sharp knife to cut off any gnarly bits of skin that won't peel.
Bring a pan of water to the boil, then add the celeriac and simmer until soft.
Drain and mash with the herbs and olive oil or dairy free spread. Set aside.
To make the lentil shepherd's pie
Heat the olive oil in a large saucepan over a medium heat. You can use an ovenproof dish if you want to save on washing up.
Add the onion and cook, stirring occasionally for five minutes. Add the diced carrot and celery, reduce the heat to low and cover. Cook for a further ten minutes until starting soften.
Add the garlic, the bay leaf, cumin and oregano and cook for a further minute.
Add the vegetable stock, the lentils, the chopped tomatoes and the tomato purée. Bring to the boil, then reduce to a simmer and cook for 10-15 minutes until the sauce has thickened. Season to taste with salt and pepper.
Preheat the oven to 180°C (fan)/200°C/gas mark 6.
Transfer the lentil mixture to an oven proof dish. Cover with the herby celeriac mash and smooth over with a fork.
Bake in the oven for 15-20 minutes until bubbling and the topping is turning golden brown.