Preheat the oven to 200C/390F/Gas 6.
Place the poussin on baking tray and cook for 10-15 minutes, or until cooked and golden.
To make the sauce, melt the butter in a saucepan. Add the onion and soften for 2-3 minutes.
Add the tarragon, rosemary, white wine and cream and reduce down for 4-5 minutes.
For the new potatoes, mix the new potatoes with the melted butter and season.
To serve, place the poussin on a serving plate and pour over the sauce. Serve with the new potatoes.