Soak the dried chickpeas in a large bowl of cold water for at least 12 hours, or overnight. The chickpeas should be submerged by at least 8cm (3 inches). They will roughly double in volume. Drain and rinse thoroughly before using. Do not use canned chickpeas for this recipe.
Preheat the oven to 150°C / 300°F (fan-assisted: 140°C / 285°F). If you have a clay pot or ceramic casserole with a tight-fitting lid, use it. A heavy cast-iron Dutch oven also works very well.
Place the drained chickpeas and roughly chopped onions in your pot. Pour in the 1.5 litres (6 cups) of cold water. The water should come about 2-3cm above the chickpeas. Do NOT add salt or olive oil at this stage. Cover the pot tightly.
Place the covered pot in the oven and bake for 4-6 hours without disturbing. After 3.5-4 hours, carefully open and check: chickpeas should be very soft and the broth cloudy. If the pot looks dry, add no more than half a cup of hot water, replace the lid, and return to the oven.
When the chickpeas are fully tender, add the salt and a few grinds of black pepper. Stir gently once to distribute. Replace the lid and return to the oven for the final 45 minutes to 1 hour. The salt will season the chickpeas from within without toughening the skins.
Remove the pot from the oven and bring it directly to the table. Uncover at the table. Pour the extra-virgin olive oil generously over the top. Serve immediately with sliced raw red onion, crusty bread, and optional lemon wedges.