Heat the oven to 200C/fan 180C/gas 6. Drizzle the olive oil over the guinea fowl and season generously, inside and out.
Heat an ovenproof frying pan over a medium-high heat and brown the bird all over until golden, then remove onto a plate. Add the lardons and onions to the pan, and cook for 5-10 minutes or until the fat has rendered from the lardons and they have started to crisp. Pour in the wine and bubble for a minute before adding the stock and mustard. Sit the guinea fowl on top of the onions and bake for 15 minutes before turning down the oven to 180C/fan 160C/gas 4 and roasting for another 30 minutes or until the juices run clear when pierced in the thigh. Remove from the oven and move the bird to a plate to rest, covered with foil, for 15 minutes.
Meanwhile, put the pan over a medium heat and pour in the cream. Bubble for 5 minutes until thickened, then stir in the tarragon or parsley.
Pour any resting juices into the sauce and serve the guinea fowl with the sauce, mashed potatoes and greens.