Lightly oil a large casserole.
Place the sliced potatoes in a large saucepan and cover with lightly salted water. Over high heat, bring to a boil, reduce heat, and simmer for 4 minutes, or until the potatoes are almost cooked through but firm, stirring occasionally. Drain, then spread the slices in the prepared casserole to cool.
Melt the butter in a skillet over medium heat. Add the onion and cook for 7 to 8 minutes, or until soft, stirring often. Add the garlic and cook for 3 to 4 minutes, or until the onions start to turn golden. Set aside.
Pour the water into a large bowl, add the sugar, and sprinkle with the yeast. Set aside for 5 minutes. Add 2 cups of flour, 1 cup at a time, and mix well with a wooden spoon. After the second cup, beat briskly 100 times. Wrap in plastic and set aside for 10 minutes.
Add the olive oil, salt, and remaining flour, 1⁄3 cup at a time, beating well after each addition, until the dough forms a ball. Turn it out onto a floured work surface. Knead with floured hands for 8 to 10 minutes, or until the dough is supple and springy, adding more flour as necessary to keep from sticking. See Finishing Steps.
By food processor:
This is suitable for a processor with a 10- to 12-cup capacity and a metal cutting blade.
Reduce the water to 1 cup plus 2 tablespoons and put it into a 2-cup measure.
Add the sugar, then sprinkle with the yeast. Set aside for 5 minutes. Add the oil and stir to blend.
Put the flour and salt into the processor and pulse to mix. Stir the liquid and, using the feed tube, add it in a 10- to 12-second-long stream, with the machine running. When the mixture forms a dough ball that rides above the blade, process (or knead)
for 20 to 30 seconds more.
Turn the dough out onto a lightly floured work surface and knead with floured hands for 30 seconds. See Finishing Steps.
Lightly oil a large bowl. Place the dough in it, rotating it to coat the surface.
Cover the bowl with plastic wrap and set aside to rise in a warm, draft-free spot for 45 to 60 minutes, or until doubled in bulk. Proceed with the recipe as directed.
Pizza Dough or your favorite, prepared as directed.
Lightly flour a counter or work surface.
When the dough has doubled, punch it down and turn it out onto the work surface. Divide in half and knead into two balls. Let rest for 5 minutes.
Preheat the oven to 425°F. Lightly dust two baking sheets with cornmeal.
Working with one ball of dough at a time, roll it into a thin, 121⁄2- to 13-inch circle.
Transfer to a baking sheet. Pinch the edge of the dough so that it’s slightly higher than the center. Layer on half of the onions, then half of the potatoes, overlapping them slightly. Sprinkle with salt and pepper, to taste. Sprinkle with half of the Parmesan cheese and half of the rosemary. Drizzle with half of the cream. Repeat for the other half of the dough.
If your oven is large enough, bake the pizzas together, on separate oven racks, for 20 to 23 minutes, or until the crust is golden brown around the edges. Halfway through baking, remove from the oven and sprinkle with the fontina cheese. Return to the oven, switching racks.
Transfer to a cooling rack briefly before serving.