Soak a large, unvarnished wooden stick suitable for cooking in cold water, preferably overnight.
Light the barbecue and allow the flames to flare up and die down, until the coals are glowing red and starting to turn white.
In a bowl, mix together the lemongrass, coriander, garlic, chillies, ginger and lime juice until well combined. Stir in 50ml/2fl oz of the vegetable oil, then add the poussin and turn to coat them in the marinade, scooping some inside each, too.
When the barbecue is ready, slide the poussins onto the stick, securing them with string.
Set the stick about 30cm/12in above the barbecue coals and cook for 30-35 minutes (depending on their size), turning every so often, until the poussins are completely cooked through and no trace of pink remains when the thigh meat is pierced in its thickest part with a skewer.
To make the dressingwhisk the egg yolk, Dijon mustard, walnut vinegar and 250ml/9fl oz water together in a bowl until combined. Gradually whisk in the remaining vegetable oil, whisking all the time, until emulsified. Season, to taste, with salt and freshly ground black pepper.
Place the red cabbage into a large bowl, then pour over 100ml/3½fl oz of the dressing and mix well.
Serve the poussins whole with a pile of the red-cabbage salad alongside.