Heat the oven to 230 degrees Celsius. Put a well-seasoned or nonstick 30 cm pizza pan or cast-iron frying pan in the oven.

Put the chickpea flour in a bowl, add the salt and pepper. Slowly add 1 cup lukewarm water, whisking to eliminate lumps. Stir in 2 tablespoons olive oil. Cover and let sit while the oven heats, or for as long as 12 hours. The batter should be about the consistency of heavy cream.

Remove the pan from the oven, pour 2 tablespoons of oil into it and swirl. Add the onions, return the pan to the oven and cook, stirring once or twice, until well browned, 6 to 8 minutes. Stir in the rosemary. Stir the onions and rosemary into the batter, then immediately pour the batter into the pan. Bake for 10 to 15 minutes, or until firm and the edges set.

Heat the griller and brush the top with 1 or 2 tablespoons of oil if it looks dry. Set the pancake a few inches from the griller and cook just long enough to brown it in spots. Cut into wedges and serve hot or warm.