Cook the Book: Fleur Burger with Truffles Recipe

Name: Cook the Book: Fleur Burger with Truffles Recipe
Ingredients: 2 pounds coarsely ground American Kobe beef, chilled; 1 teaspoon cornstarch; 1 cup Quick Chicken Stock (recipe below) or packaged low-sodium chicken broth; 2 tablespoons olive oil; Sea salt and freshly ground black pepper; 2 tablespoons ruby port; 4 brioche buns; 3 tablespoons unsalted butter, at room temperature; 1 ounce black truffle, very thinly sliced, about slices per serving (optional); 4 handfuls baby greens or sprouts; Grenadine Pickled Onions (recipe below); Fleur de sel; 1 teaspoon whole black peppercorns; 1/2 teaspoon whole coriander seeds; 1 cup red wine vinegar; 1/2 cup grenadine; 1 large (about 10 ounces) red onion, thinly sliced