Preheat the oven to 400°F (200°C). Slice the top off the garlic head to expose the cloves, drizzle with olive oil, wrap in foil and roast for 30–35 minutes until soft and golden. Let cool.
Peel and cut the Yukon Gold potatoes into even 1–1½ inch chunks so they cook uniformly.
Place the potato chunks in a large pot and cover with cold water by about an inch. Add 1 teaspoon of salt to the water. Bring to a gentle boil over medium-high heat, then reduce to a simmer and cook until the potatoes are fork-tender, about 15–20 minutes.
While the potatoes cook, squeeze the roasted garlic cloves out of their skins into a small bowl and mash to a paste with a fork.
Drain the potatoes well in a colander and return them to the warm pot. Allow them to sit for a minute so excess moisture evaporates—this helps prevent watery mash.
Using a potato ricer or masher, work the potatoes until mostly smooth. For the creamiest result, use a ricer; for a rustic texture, use a hand masher.
Warm the heavy cream and milk in a small saucepan (or microwave-safe bowl) until hot but not boiling. Add the butter to the warm liquid to melt.
Slowly add the warm cream-butter mixture to the mashed potatoes, folding gently until you reach your desired consistency. Stir in the roasted garlic paste, taste, and season with additional salt and freshly ground black pepper as needed.
Transfer the mashed potatoes to a warm serving bowl. Top with chopped chives and optional parsley. For an extra steakhouse touch, dot with a little extra butter or a drizzle of good olive oil before serving.