Place potato in a saucepan; cover with water. Bring to a boil. Reduce heat, and simmer for 25 minutes or until tender. Drain in a colander over a bowl, reserving 1 cup cooking liquid. Return potato to pan, and beat with a mixer at medium speed until smooth.
Dissolve yeast and sugar in warm water in a large bowl; let stand 5 minutes. Lightly spoon flour into dry measuring cups, and level with a knife. Add 2 cups flour, mashed potato, reserved cooking liquid, oil, rosemary, and salt to the yeast mixture; beat with a mixer at medium speed until smooth. Stir in 3 1/2 cups flour. Turn dough out onto a lightly floured surface. Knead until smooth and elastic (about 10 minutes); add enough of remaining flour, 1 tablespoon at a time, to prevent dough from sticking to hands.
Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, 45 minutes or until doubled in size. (Press two fingers into dough. If indentation remains, dough has risen enough.) Punch dough down; roll into a 14 x 10-inch rectangle. Place on a large baking sheet coated with cooking spray. Imagine a lengthwise line running through center of rectangle. Starting at imaginary line, cut 5 (4-inch-long) diagonal slits in dough on alternating sides of line. (Be careful not to cut through edge of dough.) Gently pull slits open. Cover and let rise 30 minutes or until doubled in size.
Preheat oven to 425°.
Uncover dough, and bake at 425° for 25 minutes or until browned. Remove from pan. Cool bread on a wire rack. Cut loaf in half lengthwise; cut each half crosswise into 12 pieces.
Note: This is a big, impressive loaf. But make 2 (7x5-inch) smaller loaves, if you prefer. Cut 5 diagonal slasheds into the dough; be sure not to cut to the outside border of the dough.
Using your hands, gently pull the slits apart so they remain open during baking.