Wash, scrub, and quarter the potatoes.
Place potatoes into a large stockpot and fill with water until the potatoes are just covered.
Add a tablespoon of salt and place the stockpot over medium-high heat. Boil the potatoes for 12-15 minutes, until fork-tender.
While the potatoes are boiling, place a small skillet over low heat, add the 1/2 stick of unsalted butter, the pressed garlic, and sprigs of fresh thyme. Keep the butter warm and let the garlic and thyme infuse it with lots of savory flavor.
Using a colander, drain the potatoes, return them to the pot, and mash them, or run them through a potato ricer for smoother, lump-free potatoes.
Add half-and-half, sour cream, and two sticks of butter, and use a potato masher to mix all the ingredients; continue mashing until the potatoes are smooth. Remove the fresh thyme from the melted butter and discard it.
Add the remaining butter to the potatoes and continue to mash. Season the mashed potatoes with freshly cracked black peppercorns and salt to taste.
Optional Garnish: a sprinkle of thyme leaves, chives or a pat of butter on top before serving.
Serve.