Slash skins of chicken breasts 3 or 4 times across. Mix 1/2 the mustard with a little of the wine & rub into the chicken breasts. Season. Heat butter & oil in a hot deep sided pan & when frothy, fry chicken breasts on all sides until golden. Remove from pan. Fry onion, garlic and celeriac in the same pan until lightly coloured. Add the rest of mustard & wine & deglaze the pan. Stir in chicken stock and bring to the boil. Add breasts turning down the heat, and continue cooking in the liquor until done - about 20 minutes. 5 minutes before they finish cooking add cream and tarragon and bubble hard until the cream caramalises and turns the sauce a beautiful golden colour. Serve with new potatoes and a green salad. Per serving: 593 Calories (kcal); 33g Total Fat; (53% calories from fat); 62g Protein; 4g Carbohydrate; 202mg Cholesterol; 369mg Sodium Food Exchanges: 0 Grain(Starch); 8 1/2 Lean Meat; 1/2 Vegetable; 0 Fruit; 1 1/2 Fat; 0 Other Carbohydrates Converted by MM_Buster v2.0n.