Colour the leg and breast in a hot pan with a little olive oil.
Soften the leeks, carrot and celery in a saucepan with the thyme and garlic and a little olive oil.
Transfer the guinea fowl to the saucepan and pour over the white wine.
Place the saucepan in the oven and cook for about 15 minutes at 160C/325F/Gas 3.
Halve the onion and fry slightly in some butter until lightly coloured.
Add a little chicken stock to the pan and cover with foil, roast in the oven for about 20 minutes at 180C/350F/Gas 4.
To make the purée: on a low heat cover the peas with some chicken stock and add a knob of butter, the tarragon, chervil and a little salt and pepper. Bring to the boil and then blitz with a hand blender.
When the onion is cooked remove the centre and fill with the purée. Serve with the guinea fowl.