Heat the oil in a skillet large enough to hold the turkey fillets in one layer. Over moderate heat, saute the turkey pieces, turning occasionally, until well browned. Remove to a warm place. Saute the minced shallot and garlic in the skillet juices 3-4 minutes, stirring frequently until soft.
Stir in the flour, a little at a time, to make a smooth paste and cook 1 minute, stirring; then gradually stir in a little of the chicken stock, continuing until the stock is used up and the sauce is smooth and quite thick. Return the turkey pieces to the pan, season with a little salt and fresh pepper to taste, spoon the sauce over the turkey, and sprinkle tarragon on top.
Cover and simmer over very low heat about 5 minutes more. Stir in the yogurt and heat, but do not boil. Spoon the sauce over the fillets and serve.