Squeeze the juice of 1/2 lemon into a bowl of cold water. Prepare the artichokes by trimming away all the tough, inedible portions. Cut the trimmed artichokes lengthwise into wedges about 1/2-inch thick. Drop the wedges into the bowl containing the water and lemon juice.
If any shelled fava bean is more than 1-inch in length, cut away the thick green skin that sheaths it.
Detach the leaves from the lettuce head, discarding any bruised, wilted, or discolored ones. Soak the leaves in two or three changes of water, drain, and shred them fine. You should have approximately 4 cups, but a bit more or less won=t matter too much.
If using fresh spring onions, cut off all the green tops and the root ends, then slice them very thin.
Choose a saucepan that can subsequently accommodate all the ingredients. Put in the sliced onions, the olive oil, and a large pinch of salt, and turn on the heat to low. Cook the onions, turning them over from time to time, until they have become completely wilted.
Drain the artichokes, rinse them in cold water, and put them in the pot, together with the shelled peas, fava beans, and shredded romaine lettuce. Sprinkle liberally with salt and turn over all ingredients several times to coat them well.
Put a lid on the pot and cook, always at low heat, turning the contents of the pot over from time to time, until the artichokes, peas, and beans are tender. It may take up to 2 hours, depending on the freshness and youth of the vegetables. Add 1/3 cup water whenever the cooking juices appear to be insufficient to keep the vegetables from sticking to the pot. You should need no more than 2 cups of water all together.
Taste and correct for salt, and serve warm, but not piping hot, from a shallow bowl or deep platter.
Ahead of time note: You may cook the dish 2 or 3 days in advance, refrigerating it in an airtight container. Reheat gently but thoroughly before serving, adding a tablespoon or two of water if necessary.