Fleur Burger with Truffles

Name: Fleur Burger with Truffles
Total time: 1 h 15 min
Yield: 4 servings
Ingredients: 2 pounds beef coarsely ground American Kobe, chilled; 1 teaspoon cornstarch; 1 cup chicken stock Quick, recipe below or packaged low-sodium chicken broth; 2 tablespoons olive oil; ground black pepper; sea salt; 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; pickled onions Grenadine, recipe below; fleur de sel; 1 teaspoon whole black peppercorns; 1/2 teaspoon coriander seeds whole; 1 cup red wine vinegar; 1/2 cup grenadine; red onion 1 large, about 10 ounces, thinly sliced