Preheat oven to 450° F. Using a fork, poke holes into your potatoes before wrapping them in foil and placing them on a baking sheet. Cut the garlic head in half, drizzle the open halves with olive oil, and also wrap in foil and place on baking sheet. Place the potatoes and garlic in the oven for about 60 to 70 minutes, until potatoes are all fork tender.
About 30 minutes before the potatoes are done, add milk to a small sauce pot, together with rosemary and thyme, over a low heat. Let the milk get warm over low heat, making sure that it never comes to a boil.
Once the potatoes are done, scoop out all the flesh and place through a food mill or ricer. Similarly, place the roasted garlic through a food mill or a ricer.
Add miso and melted butter and stir in until the potatoes are evenly coated. Then, add the warm milk (fish out the herbs before doing so!). Stir until potatoes are creamy and flavorful. Season with salt, white pepper, and black pepper, to taste.
Scoop out into a large bowl and use the back of your spoon to create a small divot into the middle. Add another tablespoon of unmelted butter to the divot and sprinkle with thyme leaves just before serving.