Place a pizza stone or inverted rimmed baking sheet on the lower oven rack and preheat to 500 degrees F. Make the potatoes: Combine the potatoes, garlic, sliced fennel and fennel fronds in a large pot of salted water. Bring to a boil, then reduce the heat to medium; cook until the potatoes are very tender, about 15 minutes. Drain and return to the pot. Add the olive oil and mash until almost smooth. Stir in 1 teaspoon salt and a few grinds of pepper; set aside.
Assemble the pizza: Keep the prebaked crust on the baking sheet. Scatter the taleggio and potato mixture over the crust, spreading the potato mixture slightly. Scatter the sliced fennel on top, sprinkle with the parmesan and drizzle with the olive oil. Transfer the baking sheet to the hot pizza stone and bake until the cheese melts and the crust is golden brown, 10 to 12 minutes. Remove from the oven and season with salt and pepper.
Combine the flour, salt and yeast in a large bowl. Stir in 2 cups warm water (about 105 degrees F) and 2 tablespoons olive oil with a wooden spoon until a shaggy dough forms. Cover with plastic wrap and let rise in a warm place until the dough rises slightly, about 1 hour.
Using lightly floured hands, transfer the dough to a generously floured surface and knead several times until it just starts becoming smooth and elastic, about 1 minute (do not over-knead). Transfer to a lightly oiled large bowl. Tightly cover with plastic wrap and let rise in a warm place until almost doubled in size, about 1 hour. Meanwhile, place a pizza stone or inverted rimmed baking sheet on the lower oven rack and preheat to 500 degrees F.
Brush two rimmed 13-by-18-inch baking sheets (not nonstick) with 3 tablespoons olive oil each. Place one piece of dough on each baking sheet. Prepare the pizzas one at a time: Cover the dough with plastic wrap, then press firmly with your hands to fill the baking.
Remove the plastic wrap and transfer the baking sheet to the hot pizza stone. Bake until the crust starts browning, about 10 minutes. Remove from the oven and carefully loosen the crust from the pan using a spatula. Top and bake or let cool, lightly wrap in plastic and set aside up to 8 hours.
Photograph by David Malosh
Get ahead! You can make the mashed potato mixture up to 2 days ahead; cover and refrigerate. You can bake the crust up to 8 hours ahead: Let it cool, then loosely wrap the whole baking sheet in plastic. (If you need only one pizza, wrap half of the unbaked dough in plastic and freeze up to 1 month. Thaw in the refrigerator overnight, let rest for 4 hours at room temperature and drizzle with olive oil before stretching.)
Recipe courtesy of Food Network Magazine