For the salt crust, mix the salt, herbs, eggs and water in a bowl.
Add the flour and slowly mix together until the dough is firm and elastic. Wrap the dough in cling film and leave to rest in the fridge for at least one hour.
Unwrap the dough and place onto a large, lightly floured work surface. Roll out the dough to about 0.5cm/¼in thick, or until it's large enough to wrap around the chicken (approximately 30cm/12in diameter).
For the chicken, preheat the oven to 220C/430F/Gas 8.
Stuff the cavity of the chicken with the rosemary, thyme, lemon and garlic and season the skin of the chicken with freshly ground black pepper.
Place the chicken, breast side down, in the centre of the rolled out dough. Wrap the dough around the chicken to seal completely. Pinch the edges of the dough together to seal the crust.
Turn the dough-wrapped chicken over and place on a baking tray, dough seam-side down.
Place in the oven to cook for 25 minutes. Remove from the oven and leave to rest for a further 25 minutes.
Meanwhile, to make the sauce, heat a frying pan over a medium heat until hot. Add the butter and the shallot and sweat for one minute.
Add the peas, broad beans and mousseron mushrooms and cook for 2-3 mintues.
Add the Sauternes wine and Chardonnay vinegar, increase the heat and simmer until the liquid is reduced by two-thirds.
Add the stock and simmer until the liquid is reduced by half.
Add the crème fraîche, season with salt and freshly ground black pepper and stir in the chervil.
When ready to serve, crack open the salt crust and remove the chicken. Wipe the chicken with kitchen paper to remove any excess salt.
Using a sharp knife, remove both the breasts from the carcass, taking care to keep them whole and keep the skin on.
Heat a frying pan over a medium heat until hot, add the butter and fry the chicken breasts, skin side down, until golden-brown and heated through.
To serve, spoon the pea, broad bean and mushroom sauce evenly among each of two warm serving plates. Top each with a chicken breast and serve.
