Combine 1-1/4 cups flour, sugar, undissolved yeast, and salt in a large mixer bowl. Heat water, milk, and butter until very warm (120° to 130°F). Gradually add to flour mixture. Beat 2 minutes at medium speed of electric mixer, scraping bowl occasionally. Add egg yolks and 1/2 cup flour; beat 2 minutes at high speed. Stir in enough remaining flour to make a soft dough. Cover tightly with plastic wrap; refrigerate 8 to 24 hours.
Remove from refrigerator; uncover dough and let stand 30 minutes. Divide dough in half; roll half of the dough on lightly floured surface to fit the bottom of ungreased 15-1/2 x 10-1/2 x 1-inch jelly roll pan. Place in pan, pressing dough up sides. Spread with Apple Filling. Roll remaining dough large enough to cover filling. Place over filling; seal edges together. Snip surface of dough with scissors to let steam escape during baking. Bake at 375°F for 20 to 25 minutes or until evenly golden. Cool 10 minutes; drizzle with Powdered Sugar Glaze. Let cool in pan; cut into squares. Apple Filling: Combine water, sugar, cornstarch, cinnamon, nutmeg, and salt in a large saucepan. Stir in egg yolk and sliced apples. Cook over medium heat, stirring constantly until thick, about 5 minutes. Remove from heat; let cool. Powdered Sugar Glaze: Stir powdered sugar and milk in a small bowl until smooth.