Preheat the oven to 350 degrees. Discard any excess fat from the capon. Rinse it inside and out and pat dry with paper towels. Season the inside and out with lemon juice, salt and pepper.
Thinly slice one of the onions and set aside. Quarter the other onion and place it in the cavity along with the parsley and 1 tablespoon of the olives. Brush the capon with margarine and place it on its side in a roasting pan. Scatter the sliced onions and the remaining olives around the pan. Roast the capon for 35 minutes, basting with one-third of the wine. Turn the capon on its other side and roast for another 35 minutes, again basting with a third of the wine. Turn the capon breast side up for 15 minutes, basting with the remaining wine. Turn the breast side down for another 15 minutes. The capon is ready when the drumstick juices run clear.
The total cooking time is about one hour and 40 minutes, or about 11 minutes per pound.
Remove the capon from the oven and cover it tightly with heavy foil. Let it stand for 20 minutes to let the juices flow back into the tissues. Place it on a cutting board.
Pour the liquid from the baking pan, along with the olives and onions, into a small saucepan. Place the saucepan in the freezer for about 10 minutes, so that the fat can quickly rise to the top. (This makes it easier to remove.)
To serve: Skim off the fat and reheat the sauce. Discard the onion and parsley from the cavity. Cut the breast into thin slices and serve with the sauce. Makes 10 to 12 servings.