8tbsp.unsalted butter, chilled and cut into small bits
4large Granny Smith or Golden Delicious apples, peeled, cored and cut into eighths.
4firm Bosc or Anjou pears, peeled, cored, and quartered
Making the pastry dough:
Combine the 1 ½ cups flour, 8 tablespoons butter, ¼ teaspoon salt, 1/8 teaspoon baking powder, and ½ cup sugar in a food processor, or a mixing bowl. Process 10-12 seconds, or mash with your fingertips, until the mixture has a dry, crumbly texture resembling coarse cornmeal.
Add the ¼ cup cold milk to the mixture and pulse 10 -12 times, or work together with your fingertips, just until the dough comes together in a smooth mass, but before it forms a ball. If the dough is too dry, add another tablespoon of milk and pulse or mix for a couple of seconds.
Remove the dough from the bowl, and work it into a ball with your hands. If the dough is very sticky, adhering to your fingers, coat your palms with flour once or twice and work it into the dough. The dough should be malleable and a bit tacky, but should not stick to your hands. Transfer the dough onto a piece of plastic wrap or wax paper, press it into a flat disc, wrap it well, and refrigerate for at least 1 hour. (This dough can be made well ahead and stored in the refrigerator for 2-3 days.)
Making the tarte tatin:
Preheat the oven to 400 degrees F.
Over medium heat, melt the 6 tablespoons butter in a glass stovetop-to-oven casserole or a deep cast-iron skillet. Sprinkle the ½ cup sugar evenly over the bottom of the pan, and then tightly pack in the apple and pear slices, alternating them, working from the outside edge of the pan in to the center. Fill any gaps with small slices of apple or pear.
Cook over medium-low heat for about 20 minutes, until the butter caramelizes – thickening and turning a golden amber color. Shake the casserole or pan slightly from time to time to prevent sticking and, if you are using a glass casserole, check the bottom for burning. Remove from the heat.
Roll out the pastry dough on a floured work surface to a thickness of 1/8 inch, then, using a pot cover or upside-down tart tin as a guide, cut out a 10-inch circle. Lay the pastry circle on top of the apples and pears and tuck in the edges around the fruit inside the casserole or pan. Press the dough against the sides to seal.
Bake in the center of the oven for 25-35 minutes, until the pastry is a deep golden brown.
Cool on a wire rack for 10 minutes, and then gently loosen the pastry and fruit with a metal spatula. Place a large serving plate top side down over the casserole or pan, invert the tart, and carefully transfer it – fruit side up – to the plate. Serve warm, accompanied by crème fraiche or vanilla ice cream. Serves 6.