Put water in a large bowl. Slowly add in chickpea flour, mixing it with a wire whisk. Add salt to the mixture. Cover and let stand at room temperature for 3 hours or overnight. Preheat oven to 400 degrees F. Remove the foam from the top of the chickpea mixture. Grease a baking pan, about 12 by 16-inches, with oil. Once greased, add 2 tablespoons oil. Add chickpea mixture. Use a fork to combine the oil and chickpea mixture. Sprinkle generously with pepper. Heat remaining 3 tablespoons oil in a medium nonstick pan on medium heat. Add onion and cook for 10 minutes. Top the farinata with the onion slices and bake for 20 to 25 minutes. Remove from the oven and let cool for about 10 minutes before cutting.