Combine the veal, chicken livers, nuts, herbs, seasoning and brandy together.

Lay the guinea fowls out flat on a working surface and spoon half the stuffing mixture into each one. Wrap the guinea fowls around the stuffing and sew them up with a trussing needle and thread. Place three bacon slices over the breast of each fowl and tie them on with string.

Melt half the butter in a large flameproof casserole. Add the guinea fowls and fry until they are evenly browned. Add the onion, herbs and stock and bring to the boil. Reduce the heat to low, cover and simmer for 50 minutes to 1 hour, or until the fowls are cooked through and tender.

Five minutes before the end of the cooking time, melt the remaining butter in a saucepan. Add the mushrooms and cook for 3 minutes. Remove the pan from the heat.

Remove the casserole from the heat and carefully transfer the guinea fowls to a warmed serving dish. Remove the bacon slices. Arrange the mushrooms around the fowls and keep hot while you make the sauce.

Strain the cooking liquid into a saucepan skimming any scum from the surface. Bring to the boil.

Combine the egg yolk and cream in a small bowl. Stir in about 4 tablespoons of the stock, then pour the mixture into the saucepan. Simmer gently for 3 minutes, or until the sauce has thickened and is smooth. Do not let the sauce boil or it will curdle.

Pour the sauce into a warmed sauceboat and serve at once, with the stuffed guinea fowls.