In a large pot of cold water, bring the potatoes and garlic to a boil. Salt the water and cook until the potatoes are tender about 20 to 30 minutes.
Meanwhile, heat the cream, 4 tablespoons butter, 1 tablespoon chopped sage, and the thyme sprigs over medium heat until the butter has melted. Remove from the heat and let the herbs sit in the warm cream until the potatoes are done. Remove the thyme sprigs before adding them to the potatoes.
Drain the potatoes, return the potatoes to the pot, and mash over low heat, or mash in the bowl of a stand mixer fitted with the paddle attachment. Add the herbed cream and crème fraîche.
In a skillet, melt 6 tablespoons of butter and the sage leaves over medium heat until just browned. The butter will melt, foam and froth, then begin to brown along the bottom. Whisk browned bits off of the bottom of the pan. Remove the sage leaves. Pour the browned butter into the warm mashed potatoes. Taste and season with salt and pepper.
Serve the potatoes warm, topped with the remaining 2 tablespoons butter, sage, and fresh chopped chives.