In a large mixing bowl, whisk together the chickpea flour, water, olive oil, salt, black pepper, and rosemary (if using) until smooth. The batter should be thin and without lumps.
Let the batter rest at room temperature for at least 30 minutes to allow the flour to hydrate.
Preheat your oven to 450°F (230°C).
Place a large, well-seasoned cast-iron skillet or baking sheet in the oven to heat up.
Once the oven and the skillet or baking sheet are hot, carefully remove the skillet from the oven.
Pour a little olive oil into the skillet to coat the bottom.
Quickly pour the batter into the skillet, spreading it out evenly.
Return the skillet to the oven and bake for about 20-25 minutes, or until the fainá is golden brown and crispy around the edges.
Remove the fainá from the oven and let it cool slightly before slicing.
Cut into wedges or squares and serve hot or at room temperature.
Note: Fainá is a popular dish in Uruguay, often enjoyed as a side dish or snack. It is especially delicious when served alongside pizza.
Enjoy your meal!