Wash, peel, and roughly chop the potatoes into even 1-inch chunks.
Place the potatoes in a large pot and cover with 1 inch of cold water. Stir in 1 tbsp salt. Bring to a boil.
Once boiling, cook until very tender, about 10 to 15 minutes. Drain and set aside.
Melt the butter in the same pot you used to cook the potatoes. Add the minced garlic and sauté for 1 to 2 minutes, until fragrant. Remove from the heat, pour in the milk, and stir for 15 seconds. Add the potatoes back to the pot.
Mash the potatoes with a masher. Add the sour cream, prepared horseradish (start with 2 tbsp), and 1¼ tsp salt. Continue mashing the potatoes until a creamy texture forms. Add more horseradish if desired.
Transfer to a serving bowl and garnish with Member's Mark™ Chopped Chives.