1 Trim mushrooms and finely chop. Peel shallots and chop finely. Squeeze lemon.
2 Heat butter in a pan. Sauté mushrooms and shallots over medium heat while stirring. Continue cooking over medium heat until all the liquid has evaporated. Season with a few splashes of lemon juice and season with salt and pepper. Allow to cool completely.
3 Separate egg (save white for another use). Mix ground veal with egg yolk and cream. Season with salt and pepper and refrigerate.
4 Rinse tarragon, shake dry, pluck leaves and finely chop. Set half aside, chop and mix the remaining half with the mushroom mixture.
5 Rinse chicken fillets, pat dry and, with a sharp knife, make a small horizontal incision in the upper round end of each breast.
6 Put a wooden spoon into the incision to form a pocket.
7 Put mushroom mixture in a piping bag with a long tip and pipe into the chicken breasts. Close the pocket openings with toothpicks. Cover with plastic wrap.
8 Rinse, peel and quarter potatoes. Rinse Jerusalem artichoke, thoroughly scrub with a brush and cut into pieces. Add both to milk with a pinch of salt, cover and bring to a boil. Cook over low to medium heat for 20 minutes.
9 Meanwhile, heat the oil in a non-stick pan. Season chicken breasts with salt and pepper and fry on each side. Remove and place on a baking sheet and bake in preheated oven at 200°C (fan: 180°C, gas mark 3) (approximately 400°F/convection 350°F) for 12-15 minutes. Turn off oven. Cover chicken with aluminum foil and let stand for 10 minutes in oven.
10 Stir vermouth and white wine into the pan. Add chicken stock and soy creamer and cook until creamy over medium heat.
11 Drain Jerusalem artichoke and potatoes, taking care to collect the milk. Mash vegetables and milk to create a purée. Season with salt, pepper and nutmeg.
12 Put the remaining tarragon in the sauce. Beat the sauce with an immersion blender until slightly frothy. Cut chicken into slices and serve with mashed potatoes and sauce.