Scrub potatoes with vegetable brush under cold running water. Peel potatoes and cut into chunks (about 3 inches). Place potatoes in large saucepan and add cold water to cover completely. Heat to boiling, and then reduce heat to simmer. Cover and cook until tender, about 15 to 20 minutes. Drain. Mash with potato masher or press through food mill or ricer. You should have about 3 cups. Set aside.
In large mixing bowl stir warm water and yeast together and let stand until mixture bubbles and has a foamy top. Stir olive oil and salt into yeast mixture. Add mashed potatoes and mix well. Gradually add flour, 1 cup at a time, mixing well. Oil a large bowl and turn dough into bowl. Cover loosely and let rise in warm place until doubled in bulk, about 1 hour. Lightly oil a 10x15-inch baking pan. Turn dough out into pan and press dough evenly in pan. Cover loosely and let rise in a warm place until doubled in bulk, about 1 more hour.
Preheat oven to 450°. Pull rosemary leaves from stems and chop leaves. Sprinkle top of dough with chopped rosemary and sprinkle with additional kosher salt. Using fingers make evenly spaced dents in top of dough. Bake until top is golden, about 15 to 20 minutes.
Nutritional Analysis per serving: 406 calories, 10 g protein, 68 g carbohydrates, 10 g fat (22% of calories from fat), 0 mg cholesterol, 3 g fiber, 1171 mg sodium.