Preheat oven to 375°F.

 Slice ¼ inch off the top of a head of garlic, exposing the tops of the cloves inside. Drizzle cut garlic with 1 tablespoon evoo, and wrap in foil. Roast in a 375°F oven for 40 to 50 minutes, or until the cloves are soft and lightly browned. Set aside to cool.

Place potatoes and celery root in a large pot. Cover with cold water, until it reaches 2 inches over the vegetables. Set to boil over medium high heat. Once water comes to a boil, cook for 20 minutes, or until vegetables are very soft but not breaking apart.

Drain vegetables, and return them back to the pot. Place over low heat to gently dry out vegetables in the warm pot for 3 to 5 minutes.

Transfer vegetables to large mixing bowl. Squeeze garlic from the bulb. Rice or mash vegetables and roasted garlic through a potato ricer, or use a potato masher. Do not use a mixer as the potatoes will become too starchy.

Add homemade aioli to vegetable-garlic mixture. Stir to combine. Add additional aioli until you have the texture and fluffiness you like. Season with salt and freshly ground pepper. Garnish with chopped chives, celery leaves, and fried sage.