Start by boiling a pot of salted water, enough to cover the potatoes. Add the potatoes and boil for about 15 minutes, until they are easily pierced with a fork.
While the potatoes are cooking, start the mushrooms. In a large pan over medium heat, add a glug of oil. Saute the onions with a good pinch of salt until soft, about 5 minutes. Add the garlic after a few minutes.
Then add the mushrooms, give it a good stir, then cook for about 7 minutes, until soft.
Then add the miso and tomato paste and coat the mushrooms as well as you can. Mix the arrowroot powder into 1/2 cup milk or alternative. Slowly whisk this into the mushrooms. They should become creamy and breakdown the miso and tomato paste clumps. Allow this to thicken and create a sauce, then turn heat off.
Drain the potatoes then return to pot. Add the butter, a really good pinch of salt and pepper. Mash away. Add milk if you want it creamier and more moist. Taste and adjust salt and pepper.
Plate with a big dollop of potatoes then cover with mushrooms, sauce, and a sprinkle of fresh herbs.