Preheat the oven to 425F. Line a baking sheet with parchment paper.

1. Heat 1/4 cup vegetable stock in a large pan, add the prepared onions and cook on medium heat until they begin to soften. About 5-7 minutes, add more stock to the pan if it becomes too dry.
2. Add the herbs, black pepper, celery, carrot, and garlic and cook for a further 5 minutes adding more stock as necessary.
3. Stir in the lentils, tomatoes, and tomato paste, stir well.
4. Add the stock, Tamari, balsamic vinegar, and mushrooms. Bring to the boil, turn down to a simmer and partially cover the pot. Allow to simmer while preparing the potato and cauliflower.
5. Place the potatoes, cauliflower, and turmeric in a medium-sized pot and cover with water. Bring to the boil, turn down to a simmer, cover and cook until both potatoes and cauliflower are tender, about 15 minutes.
6. Drain very well. Mash the cauliflower and potatoes with a splash of non-dairy milk, black pepper, nutritional yeast, and another pinch of dried herbs.
7. Measure 1 tablespoon of cornstarch into a small bowl. Add 2 tablespoons of the lentil mixture and stir into a paste. Pour this paste back into the lentil mixture and stir to combine. This will thicken the mixture into a glossy sauce.
8. Add the frozen peas and corn to the lentil mixture, stir well, then transfer to an oven-proof dish.
9.  Spread a layer of the cauliflower/potato mixture over the top. Place the dish on a lined baking tray and bake in the preheated oven for about 30-40 minutes, or until the topping is golden and the lentils are bubbling up around the side.
10. Place the pie under the broiler for 3-4 minutes or until it has crisped up a little.
11. Serve and enjoy.