Make the crust. Pulse the flour, sugar, and salt in a food processor. Remove the lid, add the butter, and pulse 6 to 8 times to make pea-size bits. Sprinkle the water over the mixture and pulse again, until the dough just starts to form large clumps. Turn the dough out onto your work surface and pack it like a snowball. Place it on a sheet of lightly floured plastic wrap and flatten it into a 1/2-inch-thick disk. Wrap the disk in the plastic and refrigerate.
Heat the oven to 350°. On the stove top, melt the butter in a 10-inch cast-iron or other oven-safe skillet (if you don't have one, pour melted butter into a 10-inch deep-dish pie pan or round casserole). Add the fruit and lemon juice to the skillet and stir over low heat for 1 minute. Remove the pan from the heat. In a small bowl, combine the sugar, cornstarch, and cinnamon, then stir into the fruit. Smooth the fruit with a spoon.
Roll the chilled dough between two sheets of plastic wrap to form an 11-to-12-inch circle. Peel off the top sheet and invert the pastry over the fruit. Remove the remaining plastic and tuck the edge of the pastry down between the fruit and the side of the pan. Lightly brush the top with milk and sprinkle with sugar.
Bake on the center rack until bubbly and golden brown, about 45 minutes.
Variation: MAPLE-PECAN SKILLET PIE--Replace the 1/3 cup sugar in the filling with 1/4 cup maple syrup. Mix 1 tablespoon sugar with the cornstarch and cinnamon; add to the fruit along with 3/4 cup chopped pecans.
Variation: APPLE-DATE SKILLET PIE--Replace the pears with 6 1/2 cups apples (about 6 large apples) plus 1 cup chopped, pitted dates.