Make galette dough and chill. (Can be done ahead and dough can be stored in fridge or freezer, depending on when you want to use it.)

Preheat oven to 400 degrees.

Place porcinis in a small bowl and pour over hot water to cover. Set aside.

Fill a medium saucepan with water and bring it to a boil. Add some salt and the celery, carrot, potato, Brussels sprouts and peas and cook until tender, about 4 minutes. Drain well and set aside. (You should have about 3 cups.)

Drain porcinis, reserving the liquid, which you can strain and use in place of the chicken stock or store for another purpose. Chop the mushrooms and add to the other vegetables.

In the same saucepan, melt the butter over medium heat, then add the leeks and salt and sautee for a few minutes.

Stir in the flour and rosemary sprig and cook for another couple of minutes before slowly adding the chicken stock, stirring as the sauce thickens. Then slowly add the 1/2 cup of heavy cream, stirring all the while, followed by the buttermilk. Keep stirring until sauce thickens nicely. Turn off the heat and stir in the pecorino. Let cool slightly, remove the rosemary sprig and add salt and freshly ground pepper to taste.

Stir the cooked vegetables into the sauce until well combined.

Remove galette dough from fridge and roll out on a floured surface to about 1/8" thick.

Divide the vegetable mixture evenly between two little casseroles or oven-proof bowls. Cut out two pastry rounds about 1/2" larger in circumference than the bowls, and lay over each bowl. Crimp the edges as you would a pie. Cut slits in the top and brush all over with the remaining tablespoon of cream. Sprinkle a little salt over the top if you like.

Place pot pies on a baking sheet (they may bubble over a bit) and bake until golden brown, about 30 minutes. Remove, cool slightly and enjoy right away. Once entirely cooked, may be wrapped and frozen. Reheat in a 325-degree oven, directly from freezer.

Combine the flour, salt and sugar (the sugar helps the pastry brown). The butter should be cool—not cold and hard, but not too soft either. Cut half the butter into the flour mixture and work it in lightly until the dough is roughly the texture of cornmeal. Add the other half of the butter in marble-size chunks. Work it into the dough very briefly, leaving the butter in unevenly incorporated bits. Lightly fork in the ice water just until evenly moistened. Divide and gather the dough into two balls, cover separately with plastic wrap, and knead very lightly through the plastic wrap, forming the dough into two even disks.

Refrigerate the dough and let it rest at least 1 hour. It can be kept for a day in the refrigerator and can be frozen for longer. Allow time outside the refrigerator for the dough to warm until it is just soft enough to be rolled out on a floured surface.