Peel the potatoes and cut them into evenly sized chunks. Place them in a large pot with cold water. Add a generous pinch of salt. Starting with cold water helps the potatoes cook evenly.

Bring the pot to a gentle boil over medium-high heat. Cook the potatoes for about 15 to 20 minutes. They should become very tender when pierced with a fork. Avoid overcooking them until watery.

While the potatoes cook, add butter and heavy cream to a small saucepan. Warm them gently over low heat. Do not let the mixture boil. Keep it warm until the potatoes are ready.

Drain the cooked potatoes thoroughly in a colander. Let them sit for about one minute. This allows extra moisture to escape. Dry potatoes create creamier mashed potatoes with better flavor.

Return the potatoes to the warm pot. Mash them gently with a potato masher. Continue until they become mostly smooth and fluffy. A few small pieces are fine if you enjoy a rustic texture.

Pour the warm butter and cream mixture into the mashed potatoes. Add sour cream, prepared horseradish, lemon juice and lemon zest. Stir gently until everything becomes smooth and well combined.

Stir in the grated garlic, salt and black pepper. Taste the potatoes carefully. Add another small spoonful of horseradish if you want stronger heat. Adjust the seasoning before serving.

Fold in the chopped chives and transfer the potatoes to a serving bowl. Add a small pat of butter if desired. Sprinkle fresh chives over the top. Serve while hot and creamy.