Cut potatoes (skin peeled) into equal-sized cubes and soak in cold water for 10 minutes. Remove from water and transfer potatoes into a pot and boil for about 10-15 minutes, or until soft and tender. Once cooked thoroughly, remove from pot and drain off excess water. Mash to potatoes thoroughly in a large bowl until smooth and set aside.
To make garlic miso butter mixture, melt the butter on a pan at medium high heat. Add the garlic and green onions and cook until soft and fragrant. Add the miso paste and stir until well combined. Add the milk and mix well. Let cook at medium heat for another 5 minutes.
Pour the garlic miso butter mixture into the bowl of mashed potatoes and mix until well combined. Add more warm milk for a softer consistency. Season with salt and pepper to taste and garnish with green onions.