Preheat the oven to 180C/350F/Gas 4.
For the roast poussin, heat the oil in a small ovenproof frying pan and season the poussin well with salt and freshly ground black pepper. Fry the poussin, turning frequently, until golden-brown on all sides. Transfer the pan to the oven and roast until completely cooked through.
Meanwhile, for the roast potatoes, heat the oil and butter in another small ovenproof frying pan and fry the potatoes until golden-brown on all sides. Transfer to the oven and roast for 12-15 minutes, until cooked through.
For the cashew-crusted tomato, crush the cashew nuts in a pestle and mortar and press the cut side of the tomatoes into the crushed nuts. Drizzle with olive oil and place into the oven with the potatoes, for about 6-7 minutes.
For the tarragon vinegar cream, remove the poussin from the oven and transfer to a plate to keep warm. Place the pan over a medium heat, add the white wine vinegar and bring to a simmer. Stir in the cream and heat through until bubbling. Stir in the tarragon and season, to taste, with salt and freshly ground black pepper.
To serve, place the poussin onto a serving plate and arrange the tomatoes and potatoes on the side. Pour the tarragon cream around the edge and garnish with chopped fresh parsley.