- For the crust, combine the flour and salt. Cut in the butter and shortening until the mixture is a rough, crumbly texture. Whisk the egg, lemon juice and water together, then add and blend just until the dough comes together. Shape into a disc, wrap and chill for 30 minutes.
- For the filling, dice the chicken into 1-inch pieces. Heat a large heavy-bottomed pot over medium high heat with 1 tbsp each of butter and oil. Add the chicken and sauté until lightly browned. Remove and reduce the heat to medium. Add 1 Tbsp more of butter and oil and add the onion, carrot, parsnip and celery. Sauté for 3 minutes, then add the bell pepper and green beans, sautéing 3 minutes more. Add the flour and stir to coat vegetables with it, cooking for about 5 minutes (reduce the heat if the flour begins to stick). Stir in ½ cup of chicken stock , until the flour absorbs, then add the remaining 1 ½ cups of stock, ½ cup at a time, whisking at each addition. Add the mustard, bay leaves and rosemary, and bring up to a simmer. Season lightly. Add the chicken and gently simmer, loosely covered, until the chicken is fully cooked, about 20 minutes.
- In a sauté pan over medium high heat, add the remaining 1 tbsp of butter. Add the mushrooms and sauté until any liquid has evaporated and the mushrooms are tender, about 5 minutes. Add the vermouth and simmer until liquid has evaporated. Once the filling is cooked, stir in the cooked mushrooms and season to taste. Cool the filling to room temperature. The filling can be made in advance and chilled.
- For the cheddar biscuit topping, blend the flour, baking powder and salt to combine (by hand or in a food processor). Cut in the butter and cheddar until the mixture is rough and crumbly. Add the milk and combine just until the dough comes together. Chill until ready to assemble.
- To assemble, preheat the oven to 400°F. Line a baking tray with foil or parchment paper, and arrange 8 ramekins or other individual baking dishes on the tray. Cut the pie dough into 8 pieces and, on a lightly floured surface, roll out each piece into a circle just less than ¼-inch thick. Press each piece into the bottom of each baking dish. And trim the edges if needed. Spoon in the cooled filling. Break the biscuit topping into pieces over the filling, leaving a few gaps for steam to escape and for the biscuit to expand. Bake for 15 minutes at 400°F. then reduce the heat to 375°F and bake until biscuit topping is cooked through (test by pulling apart gently with a knife), about 20 more minutes.
The Chicken Pot Pies can be made in advance, cooled, chilled and reheated for 30 minutes in a 325°F oven.