Soak the dressed possum in salted water for 12 hours. Wash it 4 times in fresh cold water. Parboil the possum for about 5 minutes. Drain and dry. Rub the possum inside and out with salt, pepper and lemon juice. Place it in a heavy casserole dish or Dutch oven and add the onions. Bring the stock/water to a boil and pour over the possum. Cover the pot tightly and cook in an preheated oven at 350ºF.for 45 minutes. Peel the sweet potatoes, turn the possum and place the sweet potatoes around it. Cook, covered for 1 hour longer or until tender. Skim off the fat and let the possum brown, uncovered for 10 minutes. While it is browning, rub the flour with the butter, make it into small balls and drop them into the pot to thicken the gravy. Baste the meat once or twice with gravy. Lift out the possum, place it on a hot platter and surround it with sweet potatoes. Serve the gravy separately. Serves 3 to 4