Heat a tbsp of the oil in a large saucepan over a medium heat and fry the onions, peppers and celery for 7–8 minutes until softened and lightly coloured, stirring frequently.
Meanwhile, whizz all the mushrooms in a food processor, in two batches, to finely chop them – take care to stop short of reducing them to a purée.
Transfer the fried veg to a bowl, add another tbsp of oil to the pan and fry the chopped mushrooms for a few minutes until they darken and throw out their juices.
Return the fried veg to the pan, add the herbs, wine, passata, Worcestershire sauce and some seasoning.
Bring to the boil, then simmer over a low heat for 15 minutes until you have a thick sauce, but with a little remaining liquid. Stir in the lentils and vinegar, and adjust the seasoning.
Transfer the mushroom and lentil mixture to a large (approx. 25 x 37cm) shallow ovenproof dish, which will allow plenty of crispy golden top, discarding the herbs.
The pie base can be prepared well in advance, in which case leave to cool before covering and chilling.
Preheat the oven to 210°C/Fan 190°C/Gas 6½, and coarsely grate the celeriac using the attachment on a food processor. Transfer the grated celeriac to a large bowl, drizzle over 3 tablespoons of oil, season and toss to coat it.
Scatter this evenly over the pie base and gently press down to level it. Toss the spring onions and cherry tomatoes with the remaining tbsp of oil and scatter over the surface. Bake the pie for 40–45 minutes until golden and crispy.