FOR CRUST: Combine flour and salt in large bowl. Add 10 tablespoons room temperature butter and cut in, using pastry blender or fingers, until mixture resembles coarse meal. Using fork, stir in water. Knead mixture in bowl gently with fingertips just until smooth dough forms. Shape into rectangle, wrap in plastic and refrigerate 1 hour. Cut 10 tablespoons chilled butter into 9 equal pieces. Arrange pieces side by side on waxed paper, forming square. Cover with second sheet of waxed paper. Pound butter several times with rolling pin to make butter more pliable. Peel off waxed paper. Break into small pieces. Roll out dough on floured surface to 9x5-inch rectangle. Turn so that 1 short side faces you. Arrange butter pieces evenly over lower 6 inches of dough. Fold unbuttered 3 inches of dough over half of buttered dough. Fold remaining 3 inches of buttered dough over, folding as for letter. Roll out to 12x18-inch rectangle. Fold short sides over to meet in center, then fold in half in same direction, forming approximately 12x4-1/2 inch rectangle. Repeat rolling to 12x18-inch rectangle and folding to 12x4-1/2-inch rectangle. Wrap in plastic; chill. (Can be made 3 days ahead. Keep chilled.) FOR FILLING: Melt butter in heavy ovenproof 10-inch skillet (preferably cast iron) over medium-high heat. Add sugar and stir until sugar melts and mixture is golden brown, about 8 minutes. Remove skillet from heat. Arrange half of apples, rounded side down, in concentric circles in skillet. Place remaining apples, cut side down, atop first layer of apples. Cover skillet. Simmer cover low heat until apples are almost tender, about 25 minutes. Uncover skillet. Increase heat to medium-low; simmer until apples are very tender and liquid is reduced to 1/4 cup syrup on bottom of pan, pressing apples down into liquid occasionally, about 45 minutes. Meanwhile, preheat oven to 400 F. Cut dough in half. Let 1 piece stand at room temperature until slightly softened, about 15 minutes.