Slightly saute the onion with the olive oil. Place pork in the pot and brown on every side. Add 1 cup of water, a little salt and simmer for about 30 minutes. Take the meat out and arrange on a platter. Place the leeks (cut into rounds), 2 cups of water and a little salt in the pot. Cook for 25 minutes at moderate temperature. Return the meat to the pot, add the tomato paste (dissolved in a little tepid water), the plums and the celery. Stir and continue cooking, until the liquids are completely absorbed. Serve warm with plenty of freshly ground pepper.