First things first: This is a spring soup. It’s meant to be made when asparagus and fava beans are in season. Just because you can find these items in the supermarket in November, when they have been flown in from the other half of the world, doesn’t mean you should give them a second thought. Eat this soup when our half of the planet is awakening from its long winter nap and you’ll feel renewed, too.
Beat the eggs in a medium bowl until frothy. Stir in the breadcrumbs, Parmigiano-Reggiano, and parsley leaves. Roll into 12 balls and set aside.
If using fava beans, bring 1 quart salted water to a boil over high heat. Add the shelled fava beans and blanch for 1 minute; immediately transfer them with a slotted spoon to a bowl of ice and water to cool. When they are cool enough to handle, gently peel the skin from the beans.
Trim off and discard the green parts of the leeks, leaving about 5 inches. Cut the white parts in 1/2 lengthwise and then into 1-inch pieces (about 3 ½ cups); rinse them well and pat them dry.
Melt the butter in a large skillet over medium heat. Reduce the heat to medium-low; add the leeks and cook for 5 minutes, or until they have softened.
Meanwhile, bring the chicken stock to a boil in a 4-quart saucepan. Trim the asparagus. Peel the bottom 1/2 of the stems if the stems are thicker than 1/3 inch; slice the asparagus crosswise into 1-inch pieces (about 11/3 cups). Clean, trim, and quarter the mushrooms (about 2 2/3 cups). Add the asparagus and mushrooms to the leeks and cook for 2 to 3 minutes more, or until almost tender. Microplane-grate the cheese (about 11/3 cups) or grate on the fine side of a box grater (about 2/3 cup).
Add the leek mixture to the chicken stock along with the favas and peas. Bring to a boil over high heat. Reduce the heat to low; add the dumplings and cook for 8-10 minutes, until they are cooked through. Stir in the cheese and serve.