Place the potatoes and garlic in a medium saucepan with enough water to cover.
Bring to a boil over high heat.
Reduce the heat to medium and cook for 20 minutes, or until the potatoes are very tender.
Add the spinach to the pan and cook for 1 minute.
Drain the potato mixture, reserving the cooking water.
Return the potato mixture to the same pan.
Add 1/2 cup of the cooking water, the sour cream, chives, salt, and pepper.
Mash until smooth.