Preheat oven to 425 degrees F.
Roast the garlic: cut off 1/4 inch of the garlic heads from the top. Do not peel. All garlic cloves must be exposed from the top. Grab some aluminum foil, enough to completely wrap a head of garlic, and place garlic on top (one per sheet of foil), cut side up. Drizzle with some olive oil, and season with salt and pepper, about 1/2 teaspoon salt and 1/4 teaspoon pepper per head. Roast at 425 F for about 30 minutes.
Meanwhile, bring a water to a boil in a large pot. Boil potatoes in salted water, until fork tender, about 15 to 20 minutes. Drain, and return potatoes to the pot.
Turn heat back on medium-low. Add butter and stir until melted. Then add half-and-half or heavy cream or whole milk gradually, and continue mashing until desired desired consistency. Season with salt and pepper as needed.
Squeeze out the roasted garlic cloves in a separate bowl. Add roasted garlic cloves to mashed potatoes, and stir to incorporate garlic into the mash. I like to leave a few chunks of garlic for extra pockets of flavor.
Transfer the garlicky mashed potatoes to a baking dish. Broil for 5 minutes, or until top is slightly golden brown. Serve warm.