Put the turnip, potato, celery root, onion, carrots, and parsley in a 2 inch deep, 3 quart ovenproof baking dish (I use an approximately 13x9x2-inch Pyrex baking dish). You should have about 6 cups of vegetables.
In a small mixing bowl, blend the chicken broth with the cornstarch. Pour over the vegetables and mix well. Add the salt and pepper and mix to blend. Dot the top of the vegetables with the butter.
Mix the flour, baking powder, and salt in a large mixing bowl and stir with a fork to blend. Put the pieces of chilled butter into the flour mixture and rub quickly with your fingertips until the mixture resembles coarse crumbs. Using a fork, slowly stir in the cream, until roughly mixed. Gather the dough into a shaggy mass and knead 5 or 6 times. Roll out the dough on a lightly floured board until it is the size of the top of the baking dish. The dough should be about 1/4 inch thick.
Place the dough on top of the vegetables. Bake for 55 to 65 minutes, until the vegetables are cooked through and the crust is browned. Test vegetables for doneness with a knife tip or skewer. Remove from the oven and serve hot.