Season the chicken thighs and, in a frying pan, seal off the chicken thighs. Seal the skin side for at least 8-10 minutes, making sure that you render the fat and get it nice and crispy. Remove from the pan and put onto a tray. In the same pan, sweat off the shallots and garlic. Add the vinegar and reduce until nearly evaporated. Add the cider and chicken stock to the pan and reduce by half. Add the chicken, half the tarragon and cream, cover and cook for 20-25 minutes until the chicken is tender.
Remove the lid, turn up the heat and add the mussels and asparagus heads. Cook for about 3-4 minutes until opened, season and squeeze juice of a lemon and add the remaining tarragon.
In a large bowl, lay the lettuce on the bottom then ladle the chicken over and the mussels and lay the asparagus on top. Bring the sauce to the boil and pour over on the plate.