Heat oven to 180C/160C fan/gas 4.
In a roomy flameproof casserole dish,
heat the oil and butter until foaming.
Season the guinea fowl all over. Spend a
good 10 mins gently frying it on all sides
until browned, then remove to a plate.
Fry the bacon in the same dish until
starting to colour, then add the carrot,
onion, celery and bay, and fry for 10 mins
until the vegetables have softened. Stir
in the lentils, pour over the Sherry and
chicken stock to just cover, and add the
tarragon. Nestle the bird back among the
lentils, breast-side up, cover with a lid
and put in the oven for 1 hr.
While the bird is roasting, make the
sauce. Bring the cream and lemon juice
to the boil and season. Remove from the
heat, add the herbs, purée with a hand
blender and set aside.
When the guinea fowl is ready,
remove from the dish and give the
lentils a good stir. Add the mustard
and a drizzle of olive oil to the lentils,
then transfer them to a serving plate.
Place the guinea fowl on top and serve
with the sauce alongside.