Step 1

Make the batter: Whisk together the chickpea flour and fine sea salt in a large bowl. Slowly pour in the water, whisking constantly to avoid lumps. Set the batter aside to rest for at least 4 hours. Whisk periodically and use a spoon to skim off any foam that rises to the surface.

Step 2

Preheat the oven and skillet: Arrange an oven rack on the bottom shelf and one on the top shelf. Preheat the oven to its highest setting, just below broil. When the oven is ready, place a 14-inch pizza pan or cast-iron skillet on the bottom rack of the oven to heat for 10 minutes. If using the rosemary, stir it into the batter. Otherwise, give the batter a final whisk.

Step 3

Pour the oil: Using oven mitts, remove the hot skillet from the oven and set it on the stovetop or a heat-proof surface. Pour in the olive oil, gently swirling the skillet if necessary to distribute the oil.

Step 4

Pour the batter: Holding a long wooden spoon by the handle, position it at a 45-degree angle so it’s just touching the surface of the oil. Slowly pour the batter over the back of the spoon in a steady stream so that the batter flows into the skillet without disturbing the oil on the bottom. Some of the oil will pool to the surface as you pour.

Step 5

Coat the batter: Using the spoon, “pull” or drag the oil on the surface in towards the center of the skillet to cover the surface of the batter (the oil may not cover the entire surface of that batter; this is fine). If using the onion, scatter the slices evenly over the surface.

Step 6

Bake, then broil: Using oven mitts, carefully return the skillet to the bottom shelf of the oven, taking care not to let the batter slosh around. Bake for 15 to 20 minutes, until the farinata is just set and beginning to turn pale gold on top. Transfer the skillet to the top shelf of the oven and turn on the broiler. Broil for 3 to 4 minutes, until the surface of the farinata is golden-brown.

Step 7

Finish, slice, and serve: Remove the skillet from the oven and sprinkle with flaky sea salt and freshly ground pepper, if using. Use a pizza wheel or knife to cut the farinata into squares or rectangles and transfer to a warmed serving plate. Serve hot.