For the pickle salad, place the vinegar, mirin and sugar into a pan over a medium heat and bring to the boil. Reduce the heat and simmer for 2-3 minutes, or until the sugar is dissolved.
Add the Thai shallots and remove from the heat. Leave to pickle for two hours at room temperature.
Drain the shallots from the liquid, reserving the pickle solution for the dressing.
Place the pickled Thai shallots, edamame beans, coriander, mint, and sliced green chilli into a clean bowl. Add two tablespoons of the pickle solution and mix well.
For the poussin, using meat scissors or a sharp, heavy knife, cut each poussin into four pieces and place into a large bowl.
Place all of the remaining poussin ingredients, except the plums, into a food processor and blend to a smooth paste.
Roughly chop the plums and add to the marinade paste.
Pour the marinade over the chicken and place in the fridge to marinate for one hour.
Remove the poussins from the fridge and heat the grill to its highest setting.
Heat a large griddle pan until hot, then add the poussin pieces, skin-side down, and cook for 3-4 minutes. Turn the poussin pieces over and cook for another 3-4 minutes.
Transfer the poussin pieces to a grill tray lined with aluminium foil and baste with any remaining marinade.
Place under the grill and cook for 12-15 minutes, basting 2 or 3 times with the marinade during cooking, until cooked through and caramelised. Test the meat by piercing with a skewer in the thickest part of the thigh. If the juices run clear the poussin is cooked. Leave to rest in a warm place for 4-5 minutes.
To serve, place the poussin onto a large serving dish. Spoon the pickled salad over the poussin and serve.