Place a 12-inch (30 cm) cast-iron pan in the oven and let heat up until ready to use.
In a bowl, add the chickpea flour, water, 2 tablespoons of olive oil, and salt, and whisk to combine.
Cover the bowl with a towel or plastic wrap and let rest at room temperature for at least 30 minutes.
Preheat the oven to broil.
Carefully remove the hot cast-iron pan from the oven, and pour in the remaining 1 tablespoon of olive oil, swirling the pan so that the bottom is fully coated.
Gently pour in the chickpea flour batter into the pan, swirling the pan so that the batter covers the entire pan.
Place the cast iron pan in the oven and broil for 5-8 minutes, until the edges are crisp and top is cooked through.
Sprinkle with flaky sea salt, pepper, and rosemary. Slice and serve.