Mashed Baked Potatoes with Chives

Arrange a rack in lower third of oven; preheat oven to 425°F. Prick potatoes all over with a fork and place on a foil-lined rimmed baking sheet. Place garlic on a piece of foil and drizzle with oil; season with salt and pepper. Fold edges of foil up and over garlic and crimp to close, creating a tight packet. Place on baking sheet with potatoes and roast until a knife slides easily through flesh of potatoes, 65–75 minutes. Let potatoes and garlic cool slightly.
Halve potatoes lengthwise, then use a fork to scrape flesh into a large saucepan (include skins if you want to add a little texture). Squeeze garlic cloves from skins into pan. Smash mixture with a potato masher until mostly smooth with only a few lumps.
Combine half-and-half and butter in a large measuring cup and pour 1 cup into potato mixture, stirring with a wooden spoon until incorporated and smooth. Stir in another cup half-and-half mixture; season generously with salt and pepper.
If serving immediately, stir in remaining half-and-half mixture and top with chives.
If making ahead, smooth potatoes to make a flat, even surface and pour remaining half-and-half mixture over (don’t stir in). Remove from heat and cover with plastic wrap.
Potatoes can be mashed 3 hours ahead. Store room temperature. Reheat over medium-low, stirring occasionally. Top with chives just before serving.