Lightly coat two 9 x13-inch (23 x 33 cm) baking pans or rimmed baking sheets with non-stick spray. In a blender, purée the chickpea flour and water. Pour the batter into a large, heavy-bottomed pot and place over medium heat. Whisk in the garlic, olive oil, salt, and pepper and continue to cook, whisking constantly, until thickened to a paste, about 15 minutes. Stir in the rosemary, then pour the batter evenly into the prepared pans. Tap the pan lightly against the counter to level, then refrigerate, uncovered, until set, at least 2 hours.