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Preheat the broiler.
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Sift the chickpea flour with the salt into a medium bowl.
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Slowly add 1/4 cup of the water, whisking constantly to form a paste. Beat with a wooden spoon until smooth. Whisk in the remaining 1/4 cup of water and let the batter stand at room temperature for 30 minutes, then stir in the rosemary.
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Heat 1 tablespoon of the olive oil in a 12" nonstick ovenproof skillet.
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Stir the batter once, pour it into the skillet and drizzle the remaining 2 tablespoons of olive oil on top.
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Cook the pizza over moderately high heat until the bottom is golden and crisp and the top is almost set, 2 to 3 minutes.
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Burst any large air bubbles with the tip of a knife.
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Sprinkle the tomato, onion, Parmesan and pepper over the top, then place the skillet under the broiler and cook until the pizza is golden and crisp, 4 to 5 minutes.
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Slide the pizza onto a work surface, cut into wedges and serve hot.