Heat the Oven - Set the oven rack to the center position. Preheat to 400ºF (204ºC).
Prepare the Garlic - Peel off the papery skin from the garlic, leaving the cloves intact. Slice ¼-inch off the top to expose the cloves. Place the garlic on foil, drizzle with olive oil, and wrap tightly. Put the wrapped garlic on a baking sheet for roasting.
Roast the Garlic - Roast until garlic until lightly browned in color, and tender when pierced with a fork, 40 to 60 minutes. Check at 40 minutes, then every 10 minutes for doneness.
Make a Garlic Paste - Remove garlic cloves by digging out the flesh with a small knife, or squeezing out the cloves. Use a knife or spoon to press into a garlic paste. Set aside.
Boil the Water - In a large pot, add the water and 1 teaspoon salt and bring to a boil.
Cook the Potatoes - Peel the potatoes and cut into 1-inch pieces. Add potatoes to a bowl and rinse under cool water to remove the starches. Drain potatoes and add to the boiling water. Cook until the potatoes are fork-tender, about 15 minutes.
Rinse the Potatoes -Drain the potatoes in a colander and rinse with hot water for 30 seconds to remove any residual starches.
Mash the Potatoes - Set a food mill, ricer, or fine mesh strainer over the pot used to cook the potatoes. This helps to keep them warm. Working in batches, press the potatoes through into the pot. If using a strainer, use the back of a spoon.
Add the Butter - Add butter and gently fold into the potatoes.
Heat the Milk - Microwave the milk for 45 seconds to warm. Whisk together the milk, ¾ teaspoon salt, and garlic paste.
Fold in the Milk -Gradually fold the milk into the potatoes in three additions, allowing it to absorb before adding more. Season with salt and pepper to taste.
To Serve - If needed, rewarm the potatoes over medium heat, stirring occasionally. Garnish with black pepper and chives.
Notes
Make it Dairy-Free: Substitute butter for olive oil, dairy-free butter, or margarine. Use non-dairy milk like cashew, almond, or oat milk. The flavor will differ slightly and may not be as creamy.
Storing: Cool the potatoes, transfer them to an airtight container, and refrigerate for up to 3 days. They can also be frozen in a resealable plastic bag for about one month.
Reheating: Warm on the stovetop over medium-low heat. Cover and reheat in the microwave on high power in 30-second intervals, stirring in between, until warmed. Frozen potatoes can be defrosted overnight, then reheated or rewarmed when frozen.