First prepare the chicken. Combine the marinade ingredients in a large bowl and mix well. Make 2-3 diagonal cuts on each side of the chicken breasts, place the breasts in the marinade and leave for anywhere between 4 and 24 hours to marinate. The longer you leave it, the more intense the flavour will be.
Meanwhile, combine the red wine vinegar, water, sugar and salt in a pan and bring to a simmer, leave until the salt and sugar dissolve.
Remove the vinegar mixture from the heat. Slice the asparagus spears in half lengthways and place in a heatproof bowl along with the other prepared vegetables.
While the marinade is still warm, pour over the top of the vegetables and cover with cling film. Set aside for at least an hour to pickle.
Test to see if the vegetables are pickled – they should be slightly tender, but still crunchy. When ready, strain and store in the fridge until ready to serve.
Remove the chicken from the marinade but retain the juices.
In a large pan, heat the oil, add the chicken and cook until golden brown. Remove from the pan and add the onions.
Cook the onions until soft, then add the juices from the marinade and the wild mushrooms. Return the chicken back to the pan, bring to a gentle simmer and cook for 30 minutes.
Remove the pickled vegetables from the fridge and allow to come up to room temperature. Once the chicken is ready, add some of the juices to the vegetables, toss to coat and place in the centre of each plate.
Slice the chicken and arrange on top, garnish with the spring onions and serve immediately.