Preheat oven to 180ºC (350ºF, Gas Mark 4).
Chop the sweet potatoes and regular potatoes into a uniform size so that they will cook evenly. Put them into a medium sized pot, cover with water and boil until soft, about 20-25 minutes.
In another medium sized pan, add 2 tbsp oil and grate in the 2 parsnips.
Add the thyme leaves and fry for a minute before adding in the lentils and then the tamari. Slowly add in the veg stock so it starts to evaporate and the lentils start to absorb it slowly. Add 1/2 a heaped tsp of black pepper and then the rest of the veg stock, letting it slowly evaporate.
While these two pots are cooking you can make the coriander pesto by adding all the pesto ingredients to a blender and blend till smooth.
Once this is done, you will see that the liquid in the lentil mixture has evaporated significantly. Make sure that there is still a decent bit of moisture so that the potatoes will absorb it. Add in the fine beans and mix through the hot lentil mixture, letting them cook in that heat.
The potatoes should now be ready (a knife cuts through them very easily). Drain the water and add 1 tsp salt and the oat milk. Then mash them all together.
Remove the bay leaves and then line the lentil and veg mix at the bottom of a casserole tray, drizzle some of the coriander pesto on top of the lentil mix and distribute the mash evenly on top. Score it with a fork to make it look nice and even.
Bake in the oven for 25 minutes until it crisps along the top.
Finish by drizzling with the remaining coriander pesto before serving. Enjoy!