Heat half the ghee in a large saucepan, season the rabbit and brown in batches, turning once. Remove from the saucepan and set aside. Add the remaining butter to the saucepan and brown the mushrooms.
Put the rabbit back into the saucepan with the mushrooms. Add the wine and boil for a couple of minutes before adding stock and bouquet garni. Cover the pan tightly and simmer gently over very low heat for 40 minutes.
Meanwhile, heat the oil in a small saucepan. Remove the leaves from the bunch of sage and drop them, a few at a time, into the hot oil. The leaves will immediately start to bubble around the edges. Cook them for 30 seconds, or until bright green and crispy. Make sure you do not overheat the oil or cook the leaves too long or they will turn black and taste burnt.
Drain the leaves on paper towels and sprinkle with salt.
Lift the cooked rabbit and mushrooms out of the saucepan and keep warm. Discard the bouquet garni. Remove the pan from the heat, mix together the cream and egg yolks and stir quickly into the stock. Return to a very low heat and cook, stirring, for about 5 minutes to thicken slightly. Do not let the sauce boil or the eggs will scramble. Season with salt and pepper.
To serve, pour the sauce over the rabbit and mushrooms and garnish with crispy sage leaves.