If you're using a ready-spatchcocked poussin, skip to Step 7. If using a whole poussin, first remove any string that's holding the poussin into shape. Trim away the spare flap of skin at the neck.
Remove the ends of the legs.
Remove the backbone. Place the poussin breast-side up on the cutting board. Carefully place a large knife into the body of the poussin with the blade facing downwards, just to one side of the backbone. Press down with the knife, cutting through the backbone.
Turn the poussin over and cut away the backbone by cutting through the back, just to the other side of the backbone. Discard the backbone.
Break the breastbone by laying the poussin flat on a board, breast-side up, and hitting the breast hard with your hand until the bone breaks.
Turn the legs so that they are facing inwards. Cut two incisions, one on either side, into the flap of skin near the poussin's tail. Thread the legs through. Tuck the wings underneath the poussin. The poussin is now ready for cooking.
For the marinade, place the cardamom pods, coriander seeds and cumin seeds into a pestle and mortar. Grind the spices to a powder. Remove the husks of the cardamom pods, leaving the seeds.
Add the turmeric to the mortar. Peel and finely chop the ginger and add to the mortar.
Peel and finely chop the garlic and add to the mortar. Pound the spices together with the pestle until the ingredients form a thick paste. Add the vegetable oil and season to taste with salt and freshly ground black pepper. Mix together well to combine.
Place the poussin in a shallow bowl and rub it all over with the marinade. Cover with cling film and marinate for at least 30 minutes.
Preheat the oven to 180C/350F/Gas 4. Heat an ovenproof cast-iron griddle pan on the hob until it is very hot. Drizzle the pan with oil, then place the spatchcocked poussin, breast-side down, onto the hot griddle. Griddle until the skin is golden, for 5-10 minutes, moving it from time to time with tongs. Turn it over and griddle the other side until golden-brown, 5-10 minutes.
Transfer it to the oven to roast, breast-side up, for about 15-20 minutes or until cooked through (when the poussin is cooked, the juices will run clear when the thigh is pierced with a skewer in the thickest part).
For the chilli jam, place the halved tomatoes over a high heat in a heavy-bottomed non-aluminium pan. Add the vinegar and water. Add the sugar and stir to combine.
Peel the garlic cloves and finely chop; add to the pan. Add the chilli flakes and mustard seeds. Cook over a medium-high heat for about an hour, until the mixture has a thick, jam-like consistency. Be careful not to let it burn. You can make the chilli jam a couple of days ahead and keep it covered, in the fridge, until ready to use.
To serve, squeeze some fresh lime juice over the poussin and place onto a serving plate. Serve with the chilli jam and some naan bread on the side, plus a green salad if you like.