1. Preheat oven to 450°F with a heavy baking sheet on rack in the center. Stir together the flour, chopped rosemary, baking powder and salt in a medium bowl. Make a well in the center, then add the water and oil and gradually stir into the flour with a wooden spoon until you have a dough. Knead it gently on a work surface 4 or 5 times.
2. Divide the dough into 3 pieces, roll out one of them on a sheet of baking paper into a 25cm round (the shape should be rustic, the dough thin). Meanwhile, cover the other two with plastic wrap.
3. Lightly brush the top with additional oil and scatter small clusters of rosemary on top, pressing in slightly. Sprinkle with sea salt. Bake on a preheated oven tray for about 8-10 minutes until the flatbread turns pale golden and browned in spots. Transfer the flatbreads to a rack to cool. Repeat with the other pieces of dough. 

Note: The flatbreads can be kept for two days at room temperature if cooled and then kept in an airtight container.