Trim the top ¼ inch off the top of the garlic bulb.
Place cut-side up on a piece of foil, drizzle with olive oil and sprinkle with salt.
Wrap the garlic in the foil and roast for 40 to 60 minutes or until the cloves are deeply golden brown and tender.
Remove from the oven and let cool slightly. Use the back of a chef’s knife or a small masher to mash the garlic into a paste before incorporating into the mashed potatoes. Set aside.
Place the potatoes and 1 teaspoon of salt in a large pot and fill with enough water to cover the potatoes by 1 inch.
Bring to a boil and cook until fork tender, about 20 minutes. Drain, reserving 1 cup of the cooking liquid.
Use a potato masher to mash the potatoes into a large bowl. Use a rubber spatula to fold in the roasted garlic, olive oil, ½ cup of the reserved cooking liquid, 1 teaspoon salt, and several pinches of pepper.
Continue folding until creamy, adding up to ½ cup more cooking liquid if desired for creamier potatoes. Serve hot with butter, and chives, if desired.
The roasted garlic can be made up to 3 days in advance and stored in the fridge until ready to use.