To make the mash, add all the ingredients to a food processor. Blitz until completely smooth, adding a little water if you want to thin it out.
To warm up - transfer to a saucepan over low heat and stir until warmed through. Taste to adjust seasoning.
For the lentil stew:
Rinse lentils well. Place in a large pot or Dutch oven and add the mushroom stock and thyme. Bring to a boil, then reduce to a simmer. Cook with lid ajar for about 25 minutes until tender. If you need to give them a little liquid top up add some boiling water from the kettle. Discard the thyme sprigs.
Meanwhile, heat a drizzle of olive oil in a large sauté pan. Add onion and mushrooms. Season lightly and cook until the onion softens and mushrooms are golden. Add garlic, tomato paste and chilli flakes. Cook until tomato paste turns a deep dark red colour and smells sweet.
Pour in the tinned tomato and deglaze the bottom of the pan. Add the cooked lentils and all their liquid. Mix everything well together. Taste to adjust seasoning Serve stew on warmed butter bean mash and sprinkle with micro herbs.