(For an alternative replace the lemon, garlic and thyme with 1 1/2 teaspoons cumin and 2 tablespoons chopped parsley)
In a mixing bowl, add garlic, thyme, and lemon juice to the yogurt and mix well. Place mixture in cheesecloth, set over a colander, set over a bowl and place in refrigerator for 12 hours.
Discard liquid and place remaining yogurt in bowl. Add salt and pepper, to taste.
Recipe courtesy of Alton Brown