Terrine de Foies de Volaille

Name: Terrine de Foies de Volaille
Ingredients: 1 pound fresh chicken livers, cleaned; 1 stick cold unsalted butter, cut into pieces; 1 cup chopped yellow onions; 2 teaspoons minced garlic; 2 tablespoons green peppercorns, drained; 1 teaspoon chopped fresh thyme leaves; 1/2 teaspoon salt; 1/2 teaspoon freshly ground black pepper; 1/4 cup Cognac or brandy; 1/4 cup heavy cream; Sel gris (gray salt) and cornichon for garnish