Heat two tablespoons of the olive oil in a medium-sized pan, add the onions and fry until lightly golden on all sides. Add the water, cover and simmer for five minutes until just tender.
Meanwhile, chop the parma ham, trimmings or the pancetta into small pieces. Uncover the pan, add the remaining oil, garlic and ham or pancetta and cook for a minute or two. Add the peas, 3/4 teaspoon salt and plenty of freshly ground black pepper. Cover and simmer gently for 15 minutes until the peas are very tender and the flavours have had time to amalgamate.
Stir in the parsley two minutes before the end of cooking. Serve as a starter or side dish.