Season the duck on both sides with 1 teaspoon of the Essence. Heat a large heavy pot or Dutch oven over high heat. Add the duck, skin side down, and immediately lower the heat to medium. Sear until the skin is browned, 5 to 6 minutes. Turn the duck and cook for 3 minutes. Transfer the duck to a plate and let cool. Discard the duck skin and cut the meat into 1/2-inch cubes.
Pour off all but 2 tablespoons of the duck fat from the pot and return the pot to medium-high heat. Add the tasso, onions, carrots, celery, and salt, and cook, stirring often, until softened, about 5 minutes. Add the shallots and cook, stirring often, until softened, about 1 minute. Add the garlic and stir until fragrant, about 30 seconds. Add the sweet potatoes and cook, stirring constantly, until beginning to soften, about 5 minutes. Add the stock, bay leaves, and the remaining 2 teaspoons Essence and the duck and bring to a boil. Reduce the heat to medium-low and simmer, uncovered, for 15 minutes.
Whisk 6 tablespoons of the heavy cream and the flour in a small bowl until smooth, then whisk into the soup. Increase the heat to medium-high. Add the remaining 1/2 cup cream and cook at a brisk simmer, stirring often, until the sweet potatoes are tender, about 15 minutes. Discard the bay leaves.
Serve hot ladled into bowls.
Yield: 6 to 8 servings