Prepare the potatoes: peel them (optional) and cut into even 1- to 1½-inch chunks so they cook evenly.

Place potatoes in a large pot and cover with cold water by about an inch. Add a generous pinch of salt, bring to a boil, then reduce heat and simmer until very tender, about 15–18 minutes.

While potatoes cook, warm the milk and heavy cream with the minced garlic and butter in a small saucepan over low heat until butter melts and mixture is warm but not boiling. This infuses the dairy with garlic flavor and prevents cooling the potatoes when mixed.

Drain the potatoes well and return them to the hot pot for a minute over low heat to evaporate excess moisture. This helps keep the mash fluffy rather than watery.

Mash the potatoes using a potato masher, ricer, or food mill until mostly smooth—avoid overworking if you want them fluffy rather than gluey.

Slowly add the warm milk-cream-butter mixture to the potatoes, stirring gently until you reach the desired consistency. You may not need all the liquid.

Stir in the grated cheddar and Parmesan a little at a time until fully melted and incorporated. Taste and season with salt and freshly ground black pepper.

Transfer to a serving bowl, sprinkle with chopped chives and any optional toppings like crisp bacon or caramelized onions. Serve immediately while hot and melty.