1. For the dumplings: Whisk together the flour, baking soda, baking powder, and salt. Cut in the butter until the mixture resembles coarse bread crumbs. Stir in the herbs, if using. Cover and refrigerate this mixture while you're making the pot pie filling.
2. For the pot pie filling: Melt the butter in a 3-quart saucepan over medium heat. Whisk in the flour and cook for 1 minute.
3. Add the stock 1/2 cup at a time, whisking it into the roux to prevent lumps. When all the stock and/or gravy is added, season with the thyme, bay leaf, salt, pepper and Worcestershire sauce. Simmer the sauce for 15 minutes, then stir in the meat and vegetables.
4. Return the filling to a simmer, and transfer to a 4-quart baking dish with a lid. Preheat the oven to 350°F.
5. To assemble: Once the hot filling is in the dish, whisk the buttermilk and egg together, and add, all at once, to the dry mixture.
6. Stir together until evenly moistened.
7. Scoop the batter on top of the simmering liquid, leaving space between the dumplings (they'll almost double as the cook). Put the lid on top, and bake at 350°F for 25 to 30 minutes.