Per Ottenlenghi's notes to this recipe, "You can keep the vegetables left over from making stock and serve them deep-fried, or even as they are, with mayonnaise or aioli.
Heat up the olive oil in a large pot. Add all the vegetables and garlic and sauté for a few minutes until they color lightly. Add the herbs, spices, and prunes and cover with cold water. Simmer for up to 1-½ hours, skimming the surface and adding more water when needed so that at the end of the cooking you are left with enough liquid for four portions.
Strain the broth through a fine sieve into a clean pan. Add some of the carrots and some celery or celeriac, if you like, as well as some torn chard leaves. Set aside ready for reheating.
Cook the potato, parsnips and garlic in plenty of boiling salted water until soft; drain well. Wipe dry the pan in which the vegetables were cooked and put them back inside. Add the butter and sauté on medium heat for a few minutes to get rid of the excess moisture. While hot mash them with a potato ricer or masher. Add the flour, semolina, egg, and some salt and pepper and mix until incorporated. Chill for 30 to 60 minutes, covered with plastic wrap.
Reheat the broth and taste for seasoning. In another pan, bring some salted water to a light simmer. Dip a teaspoon into the water and use it to spoon out the dumpling mix into the water. Once the dumplings come up to the surface, leave to simmer for 30 seconds, then remove from the water with a slotted spoon.
Before ladling the hot broth into bowls, add some torn chard leaves into the pot and cook for about five minutes. Then, ladle the hot broth into bowls. Place the dumplings into the broth, garnish with parsley and serve immediately.