Pour the boiling water over the shiitakes in a small bowl. Cover and leave to hydrate for 20 minutes. After 20 minutes, remove the shiitakes, reserving the water (which will become our stock).
Place the hydrated shiitakes in a food processor and pulse blitz lightly until the shiitakes are chopped roughly into mince like constistency. Set aside.
Preheat the oven to 200c.
In a large saucepan, add three tablespoons of vegetable oil and bring to a medium heat. Add the fennel seeds and toast for a minute. Add the onion and fry for a few minutes. Add the garlic and carrot and fry for another three minutes. If it starts to stick, add a tablespoon of water.
Throw in all of the minced shiitakes and the lentils along with the miso, marmite, soy sauce, herbs and spices and stir for a few seconds.
Add the flour and bouillon. Stir to combine, then pour the mushroom water into the pan through a fine sieve or tea strainer. Stir well and cover with a lid. Leave to cool, stirring occasionally, for around 10 minutes or until the sauce has thickened slightly.
Pour the hot filling into an oven proof casserole dish and top with the mash. Drizzle with a little oil and pop in the oven for around 15-20 minutes or until the top is golden and crispy and the sauce is bubbling around the sides.