How to Make It
Arrange the duck breasts in a single layer in a glass or ceramic baking dish. In a food processor, pulse the kosher salt with the sugar, cilantro, basil, garlic and 1 teaspoon of chives until finely ground. Rub the salt all over the duck breasts and refrigerate for 4 to 6 hours.
Fill a bowl with ice water. In a pot of salted boiling water, cook the fava beans until just tender, 3 to 4 minutes. Using a slotted spoon, transfer the favas to the ice bath. Add the peas to the pot and cook until bright green and tender, 5 to 7 minutes; drain and transfer to the ice bath. Drain the fava beans and peas, then pinch the favas out of their skins; discard the skins.
Preheat the oven to 400°. Wipe the curing mixture off the duck breasts. If they have been curing for over 4 hours, rinse them, then pat dry. In a large ovenproof skillet, cook the duck breasts skin side down until golden brown and the fat has rendered, 7 to 8 minutes. Turn the duck breasts skin side up and transfer the skillet to the oven. Roast the duck for about 15 minutes for medium-rare meat; an instant-read thermometer inserted in the thickest part of the breasts should register 135°. Transfer the duck to a cutting board and let rest for 10 minutes.
Meanwhile, in a medium saucepan, heat the olive oil. Add the asparagus and cook over moderate heat, stirring, until tender, 5 to 7 minutes. Add the spring onions and cook for 2 minutes, then stir in the fava beans, peas, chicken stock and milk and bring to a simmer. Season with sea salt and black pepper.
Thickly slice the duck breasts and arrange in shallow bowls. Spoon the vegetables on top and ladle the broth around the duck. Garnish with chives, chive blossoms and chervil and serve.