Roast the garlic: Preheat oven to 375°F. Slice off the top ¼ inch of the garlic head to expose the cloves. Drizzle with olive oil, wrap tightly in foil, and roast for about 1 hour, until soft and golden. Once cooled slightly, squeeze the roasted garlic cloves from their skins.

Cook the potatoes: Place the quartered potatoes in a large pot or Dutch oven and cover with cold water. Add 1 tsp salt and bring to a boil. Reduce heat and simmer until tender, about 15 minutes. Drain well.

Warm the dairy: In a small saucepan or microwave-safe bowl, gently heat the butter and half-and-half together until the butter is fully melted.

Mash the potatoes: Return the drained potatoes to the pot. Add the roasted garlic cloves and the warm butter–half-and-half mixture. Mash until creamy, leaving some small lumps for texture. Season with additional salt and freshly ground pepper to taste.

Serve: Transfer to a serving dish, top with extra butter, and sprinkle with fresh herbs if desired. Serve warm.