Step 1Heat the oven to 325 degrees. Cut the top of the pointy tip off the garlic (no peeling necessary), just enough to expose the tips of the cloves. Place the garlic on a small sheet of foil and drizzle a little olive oil over the cut end of the garlic head, then sprinkle a pinch each of salt and pepper. Seal the garlic head in the foil. Place the garlic in the center of the oven (place the foil directly on the rack; you don't need to put it on a baking sheet) and roast the garlic until the cloves have softened and the garlic is aromatic, 45 minutes to an hour (the garlic can be roasted while you cook the potatoes). Remove from the oven and set aside to cool.
Step 1Place the potatoes in a pot with enough water to cover by 1 inch and add 1 1/2 tablespoons salt. Cover and bring to a boil, cooking until the potatoes are soft and a knife easily pierces the potatoes, 30 to 50 minutes, depending on the size of the potatoes. Drain the water out of the pot.
Step 2Add the butter to the pot, and squeeze the soft roasted garlic cloves out of their skins into the pot. Use a potato masher to coarsely smash the potatoes and garlic together. Pour over the heavy cream and sour cream and continue to mash to combine (for a coarser texture, stir in the heavy and sour creams with a spatula or wooden spoon). Season the potatoes with 3/4 teaspoon salt and several grinds of pepper.
Step 3Taste the potatoes, adding additional heavy or sour cream if desired, and adjusting the seasoning if needed. Serve immediately.