I've streamlined Julia's recipe by combining the variation in Mastering the Art of French Cooking with the master recipe. I have also rewritten it to correspond more closely to the recipe as it is cooked in Julie & Julia. If you prefer, you can brown the chicken breast first in the butter, but I urge you to try it this way!
For the chicken:
• 4 supremes (boneless chicken cutlets)
• ½ teaspoon lemon juice
• big pinch pepper (Julia says white, but black is OK)
• 4 tablespoons butter
• ½ a 10 ounce package white button mushrooms, trimmed and quartered
• 1 tablespoon minced shallot
• a round of parchment paper cut into a 10-inch round, buttered on one side
For the sauce:
• ¼ cup brown stock (preferably reconstituted demiglace), or chicken stock
• ¼ cup port or Madeira
• 1 cup heavy cream
• salt and pepper
• lemon juice as needed
• 2 tablespoons fresh minced parsley
Heat oven to 400°. Rub the supremes with drops of lemon juice and season lightly with salt and pepper. Heat the butter in a 10-inch sauté pan with an ovenproof handle until it foams. Quickly roll the supremes in the butter, lay the buttered paper over them, cover the pan and place in the hot oven.
After 6 minutes, press the top of the supremes with your finger. If still soft, return to the oven for a moment or two, until the meat is springy to the touch. Transfer the supremes to a warm platter, loosely covered with foil, while you finish the dish.
Place the pan over a medium high heat on the stovetop, and add a little more butter if it seems dry. Add mushrooms and season lightly with salt and pepper. Sauté until mushrooms are golden brown, stirring frequently (they'll release their liquid before they start browning). In the last minute of cooking add the shallots to the pan.
Add the stock and the port to the pan and boil down quickly (reduce) until syrupy. Add the cream (use less if you want) and boil down again over high heat until the cream has thickened slightly. Taste for seasoning, adding drops of lemon juice, salt and pepper, as needed. Pour the sauce over the supremes, sprinkle with parsley, and serve immediately.