Add the onions to the pan and fry for 2 3 mins. Spoon over the chicken, then top with the tomatoes and asparagus. Set aside while you prepare the sauce. Put the sugar and vinegar into the pan. Stir over medium heat until the sugar is a dark caramel colour, then add the stock. Bring to the boil and simmer for 1 min. Whisk into the milky sauce until blended, season, then add the tarragon. Spoon sauce over the chicken and veg, sprinkle with crumbs and cheese, then bake for 2. How To Use Tarragon© 2017