Thaw frozen peach slices, if using. Do not drain. In a Dutch oven combine peaches, 1 cup granulated sugar, water, cornstarch, and cinnamon. Cook and stir until mixture is thickened and bubbly. Keep warm.
For pecan filling, stir together brown sugar and melted butter or margarine. Add pecans; toss to mix. Set aside.
For dough, in a large mixing bowl stir together flour, 2 teaspoons granulated sugar, baking powder, baking soda, and salt. Cut in shortening until mixture resembles coarse crumbs. Make a well in the center; add buttermilk. Stir just until dough clings together.
Turn dough out onto a lightly floured surface. Knead gently for 10 to 12 strokes. Roll dough into a 12x8-inch rectangle; spread with pecan filling. Roll up from one of the long sides. Cut into twelve 1-inch-thick slices.
Transfer hot peach mixture to a 3-quart rectangular baking dish. Place slices, cut side down, on top of the hot peach mixture. Bake in a 400 degree F oven for 25 to 30 minutes or until topping is golden. Serve warm with half-and-half or light cream, if desired. Makes 12 servings.