Step 1

Preheat oven to 350°. Cut ½" off top of each head of garlic to expose just the tops of the cloves inside. Place on a 12"-square piece of parchment paper or foil. Drizzle with oil and season with salt. Drizzle 1 tsp. water over.

Step 2

Bring edges of parchment up and over garlic and fold together to make a packet and seal. Place on a small rimmed baking sheet and bake until very tender, 60–75 minutes.

Step 3

Let garlic sit until cool enough to handle, then squeeze out cloves into a medium bowl. Add butter and mash together into a paste with a wooden spoon or stiff rubber spatula. Add miso and mix well. Season garlic-miso butter with salt and pepper; set aside.

Step 4

Peel and quarter potatoes. Place in a large pot and pour in water to cover by 1"; season generously with salt. Bring water to a boil over medium-high, then reduce heat and simmer gently, stirring occasionally, until potatoes are very tender (a tester or paring knife should easily slide into flesh), about 20 minutes from the time water starts to simmer. Drain potatoes and let sit 5 minutes to dry out; reserve pot.

Step 5

Bring cream to a simmer in reserved pot over medium-high. Remove from heat and return potatoes to pot. Set aside about 3 Tbsp. garlic-miso butter for serving and add remaining garlic-miso butter to pot. Using a potato masher (or use a potato ricer or food mill if you prefer a silkier texture), smash potatoes until mostly smooth; taste and season mashed potatoes with salt.

Step 6

Transfer mashed potatoes to a large shallow bowl. Top with reserved garlic-miso butter and season generously with more pepper.Do ahead: Mashed potatoes can be made 2 days ahead. Cover and chill potatoes and reserved garlic-miso butter separately. Reheat potatoes over medium, stirring often and adding ¼ cup milk or water to thin if needed.