For the pastry:
- 1 1/2 cups flour, plus some for rolling out the dough
- Fine sea salt
- 1 1/2 teaspoons ground sage (about a 1/3 palmful)
- 3 tablespoons butter, cut into small pieces
- 1/2-2/3 cup whole milk or half-and-half
For the filling:
- 2 bone–in, skin-on chicken breasts (about 2 1/2 pounds)
- 2 carrots, peeled and coarsely chopped
- 2 ribs celery, coarsely chopped
- 1 onion, quartered
- 1 large fresh bay leaf
- 1 herb bundle of parsley and thyme
- 1 tablespoon extra virgin olive oil (EVOO)
- 3 tablespoons butter
- 1 pound button mushrooms, quartered
- 2 large shallots, finely chopped
- 1/3 pound green beans, trimmed and cut into 1-inch pieces on the bias
- 2 rounded tablespoons flour
- 1/2 cup dry white wine
- 1/4 cup half-and-half or heavy cream
- 2 tablespoons Dijon mustard
- 3 tablespoons fresh tarragon, chopped
- A few grates of nutmeg
- 1 egg, beaten with a splash of water
For the pastry, place the flour in a mixing bowl and season lightly with salt, about 1/4 teaspoon. Add the sage, butter and milk or half-and-half, and combine to form dough.
For the filling, poach the chicken in a pot with the carrots, celery, onion, bay leaf, herb bundle and salt. Add enough water to just cover the chicken. Bring to a gentle boil; reduce the heat and simmer for about 40 minutes. Remove the breast and cool to handle. Strain the stock and keep at a low simmer. Remove the chicken from the bones, discard the skin and bones and pull or chop the chicken into bite-size pieces.
Heat the EVOO, one turn of the pan, in a large, deep skillet or Dutch oven over medium-high heat. Add the butter and melt into the EVOO, then add the mushrooms and brown until golden, 15 minutes. Season the mushrooms with salt and pepper; add the shallots and stir for 2 minutes more. Add the beans and toss together. Sprinkle the flour over the pan, stir for a minute more, then add the wine and reduce for 1 minute. Ladle in about 3 cups of warm chicken stock and let thicken a bit. Stir in about 1/4 cup heavy cream, add the tarragon and stir in the Dijon. Season with a few grates of nutmeg; add the chicken and combine.
Pre-heat the oven to 400°F.
Transfer the chicken to a casserole dish. Roll out the dough and trim to fit the casserole. Cut a few slits in the dough to allow the steam to escape and brush with egg wash. Bake until deeply golden, 30-35 minutes.