In a mixing bowl, whisk together the flour and the water;
Let the dough sit in the fridge for about 1 hour. The traditional recipe calls for it to be refrigerated covered in plastic wrap for 4-5 hours; the more it rests, the more the flour is absorbed by the water, and the better it tastes;
Take it out of the fridge and remove the foam, which has come to the surface, with a slotted spoon, and stir it until combined. This step helps the surface get golden brown;
Add the finely chopped rosemary, salt and pepper to the batter; stir it well with a whisk until combined;
Preheat the oven to 400-450°F (220-250°C) and lightly grease a 12-inch (30 cm) sheet pan with oil;
Farinata must be very thin, less than a centimeter (0.39 inch), so use two baking pans or make 2 batches;
Put the baking sheet into the oven for a couple of minutes to warm it up;
Then pour the mixture slowly into the pan, place it on the middle rack and bake for 10 minutes. Then place the pan on the upper rack and bake it for another 5 minutes so the surface gets golden and crispy.
Bake it for another 10 minutes with the oven door slightly open to eliminate any moisture.
Sprinkle pepper and coarsely chopped rosemary on the top; you may also season it with a thinly sliced onion to your taste.