Sift the chickpea flour with the salt into a medium bowl.
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.
Heat 1 tablespoon of the olive oil in a 12" nonstick ovenproof skillet.
Stir the batter once, pour it into the skillet and drizzle the remaining 2 tablespoons of olive oil on top.
Cook the pizza over moderately high heat until the bottom is golden and crisp and the top is almost set, 2 to 3 minutes.
Burst any large air bubbles with the tip of a knife.
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.
Slide the pizza onto a work surface, cut into wedges and serve hot.