Mix the flour and salt. Gradually add the water, whisking vigorously to eliminate lumps, and let it rest for 1 hour.

Preheat the oven to 500 F (260 C).

Whisk the batter again to obtain a thick cream consistency.
Pour the oil into an 8″ (20 cm) non-stick pan, then add the mixture.

Sprinkle the mixture with the chopped onion. Place the pan in the middle of the oven and bake for 5 minutes. Add the rosemary and bake for another 20 to 30 minutes, or until the top is golden brown and crispy.
Remove from the oven and let it cool in the pan. Serve the farinata hot or at room temperature, sprinkled with black pepper and drizzled with a stream of oil, to taste.