Roast the Garlic: Cut the heads of garlic in half crosswise (so you have 4 halves) and place them cut-side up on a small baking dish. Cover tightly with foil, and roast for 30 minutes at 425F. Remove from the oven and let cool.
When the roasted garlic is cool, squish the cloves into a small dish and mash lightly with a fork to break up any large pieces of roasted garlic.
While the garlic cools, make the mashed potatoes. Place the diced potatoes in a large pot and cover them with water by about 2 inches. Add a tablespoon or so of salt to the water to flavor the potatoes.
Turn the heat to high to bring the water and potatoes to a boil, then boil the potatoes for 10 minutes, uncovered, until the potatoes are fork tender. Drain the cooked diced potatoes and return to the hot pot off the heat.
Using a spoon or potato masher, add the salt, half and half, butter, and mashed roasted garlic to the cooked, drained potatoes. Combine gently and taste; add more salt until the potatoes are very flavorful or pepper if you'd like. If the potatoes are too thick, splash in more half and half or some milk until they thin out to your preferred consistency.
Serve immediately, or keep at room temperature for a few hours until ready to serve.