For the matzo balls:
3/4 cup matzo meal
1/4 teaspoon kosher salt
1 teaspoon baking powder
6 large egg whites, lightly beaten
3 tablespoons vegetable oil
3 tablespoons low-sodium chicken broth
For the soup:
1/2 pound shelled fresh fava beans or shelled fresh lima beans (or 1 2/3 cups defrosted frozen), or a combination
3 medium leeks
1/2 pound asparagus (about 1/2 bunch), tough ends discarded (peel the stalks if thicker than 1/3 inch)
2 tablespoons vegetable oil
1/2 pound small white mushrooms, trimmed and quartered
10 cups low-sodium chicken broth
1 cup shelled fresh or defrosted frozen green peas
Kosher salt and ground black pepper
Chopped fresh dill, to garnish
To make the matzo balls, in a large bowl stir together the matzo meal, salt and baking powder. Add the egg whites, vegetable oil and chicken broth, then stir until well combined. Cover and chill for 30 minutes.
While the matzo mixture cools, prepare the vegetables. If using fava beans, in a large saucepan bring 1 quart of salted water to a boil over high heat. Add the shelled fava beans and blanch for 1 minute. Use a slotted spoon to immediately transfer them to a bowl of ice water to cool. When they are cool enough to handle, gently peel the skins from the beans. If using lima beans, this step can be skipped.
Trim off and discard the green parts of the leeks, leaving about 5 inches. Cut the white part in half lengthwise, then slice into 1-inch pieces (about 3 1/2 cups). Rinse them well and pat them dry. Cut the asparagus crosswise into 1-inch pieces.
In a large saucepan over medium, heat the oil. Reduce the heat to medium-low, add the leeks and cook, stirring occasionally for 5 minutes, or until they have softened.
Add the asparagus and mushrooms to the leek mixture and cook, stirring occasionally, 3 to 4 minutes more, or until almost tender. Transfer the vegetables to a bowl and set aside.