In a large pot, cook bacon over medium heat until crisp. Remove bacon and reserve.
Reduce heat to medium low and cook leeks until soft, about 5 minutes.
Add potatoes, chicken stock and thyme, cover loosely and simmer until potatoes are tender, about 20 minutes.
Remove from heat and purée. Strain soup (optional) and return to medium heat. Stir in sour cream and season to taste.
Serve bowls of soup garnished with grated cheddar, reserved bacon and a sprinkle of chives or green onion.