Make seasoned flour with salt, pepper, dry mustard and cayenne pepper.
In a food processor finely mince the leg meat, add the egg white and seasoning and blend to a paste. Flatten the fillets and flour lightly. Lay a piece of bacon cut to fit on one fillet, spread a layer of the mousseline of rabbit and egg white onto the bacon and lay your sorrel of sage leaves on top of this. Place the other rabbit fillet on top to make a parcel.
In a heavy-lidded frying pan melt the butter and gently fry the onion, add the rabbit parcel and brown, turning carefully. Season, pour on white wine, cover and cook gently for 10 minutes turning the parcel halfway through.
Check that the rabbit is tender and cooked through, and serve with garlic-pureed potatoes and a green vegetable.