Use a ricer or 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, rosemary, and chives, if desired.