Put the guinea fowl into a saucepan with the carrots, onions, parsley, peppercorns and stock. Bring to the boil, then reduce the heat, cover and simmer for 45 to 55 minutes until the guinea fowl is tender and the juices run clear when the thigh is pierced with a knife.
Transfer the guinea fowl to a plate and leave to cool for 20 minutes. Meanwhile, strain the stock into a measuring jug. There should be 300ml (10 fl oz); if not, make it up to this amount with water.
Heat the oven to 180°C (350°F, gas mark 4). When the guinea fowl is cool enough to handle, remove and discard the skin, then cut the meat from the bones and chop or roughly tear it into large chunks.
Heat the oil in a large frying pan, add the mushrooms and stir-fry them over a high heat for 2 to 3 minutes until lightly browned. Remove them from the heat and set aside.
Melt the fat spread in a saucepan over a medium heat, stir in the flour then gradually stir in the cooking liquid from the guinea fowl. Bring to the boil and stir continuously until the sauce thickens. Reduce the heat and simmer for 3 minutes, stirring occasionally.
Remove the sauce from the heat and stir in half the lemon zest, the lemon juice and the yoghurt. Season with salt and pepper, then stir in the guinea fowl and mushrooms.
Pour the fricassée into an ovenproof serving dish, cover and bake for 25 to 30 minutes until heated through.
Stir the fricassée to blend in any mushroom juices, then sprinkle it with the rest of the lemon zest and the chopped parsley. Serve garnished with sprigs of parsley, and toast triangles if you like.
Variation: you can substitute a small chicken for the guinea fowl.