Preheat your oven to 400°F (200°C).

Slice off the top of the garlic bulb to expose the cloves inside. Place the bulb on a piece of aluminum foil, drizzle with olive oil, and wrap it tightly in the foil.

Roast in the preheated oven for about 30-35 minutes, or until the garlic cloves are soft and caramelized.

Remove from the oven and set aside to cool.

While the garlic is roasting, place the peeled and quartered potatoes in a large pot of salted water.

Bring to a boil and cook until the potatoes are fork-tender, about 15-20 minutes.

Drain the cooked potatoes and return them to the pot. Mash them with a potato masher or fork until smooth and creamy.

While the potatoes are cooking, heat a small amount of olive oil in a skillet over medium heat.

Add the chopped onion and sauté until softened and translucent, about 5-7 minutes.

Once the roasted garlic has cooled slightly, squeeze the softened cloves out of the skins and add them to the mashed potatoes.

Add the sautéed onion, butter, sour cream, grated Parmesan cheese, milk, salt, and freshly ground black pepper to the mashed potatoes.

Use a potato masher or hand mixer to mash and mix the ingredients together until well combined and creamy. Adjust the consistency with additional milk if needed.

Taste the mashed potatoes and adjust the seasoning with salt and pepper to taste. Add more roasted garlic if desired for a stronger garlic flavor.