Step 1

Arrange a rack in lower third of oven; preheat oven to 425°. Scrub 2 lb. russet potatoes and 2 lb. Yukon Gold potatoes and prick all over with a fork. Place on a foil-lined rimmed baking sheet.

Step 2

Halve head of garlic crosswise (as if you’re cutting through the equator) and place on a piece of foil. Drizzle with 1 Tbsp. 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.

Step 3

Halve potatoes lengthwise. Using fork, 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.

Step 4

Cut 6 Tbsp. butter into ½" pieces and combine in a small saucepan with 2 cups half-and-half. Heat over medium-low, swirling, until butter melts and cream is warm but not boiling (it can be simmering gently around the edges).

Step 5

Pour about half of half-and-half mixture into potato mixture, stirring with a wooden spoon until incorporated and smooth.

Step 6

If serving mashed potatoes immediately, stir in all of remaining half-and-half mixture and season generously with salt and pepper. If making ahead of time, stir in all but ½ cup of half-and-half mixture, season with salt and pepper, and smooth potatoes to make a flat, even surface. Pour reserved ½ cup half-and-half mixture over and don’t stir (this is to prevent the potatoes from drying out and forming a film). Cover with plastic. You can do this part up to 3 hours ahead.

Step 7

Thinly slice 8–10 chives.

Step 8

To serve, warm potatoes over low heat, stirring well with wooden spoon. At this point, if you made it ahead of time, mix in half-and-half mixture floating on top of potatoes.

Step 9

Transfer mashed potatoes to a platter and top with chives.