Preheat oven to 350 degrees Fahrenheit. Place a 1-quart capacity au gratin dish or other casserole on a baking sheet and set aside.
In a large skillet over medium-high heat, sauté the beef and onion until the beef is no longer pink. Stir frequently to prevent sticking. Season with salt and pepper.
Stir in the mushrooms, thyme and 2 tablespoons flour. When the flour is all stirred in, add the Worcestershire sauce and deglaze the pan with the chicken stock, bring mixture to a boil, then simmer 10 minutes, stirring occasionally.
Meanwhile, make the dumpling topping by putting the 2/3 cup flour, 1/2 teaspoon baking powder and 1/2 teaspoon salt in a mixing bowl and rub in the lard. Stir in the milk and thyme. Set aside.
When the filling has simmered 10 minutes, transfer it to the au gratin dish and top with rounded tablespoonfuls of topping. Bake in the middle of the oven about 25 minutes, or until dumplings are cooked through.