Oxtail Soup with Onions and Barley

Name: Oxtail Soup with Onions and Barley
Ingredients: 3 tablespoon vegetable oil, divided; 4 pounds oxtails or beef shanks; Kosher salt and freshly ground black pepper; 3 large shallots, coarsely chopped; 2 medium carrots, peeled, coarsely chopped; 2 celery stalks, coarsely chopped; 4 garlic cloves, chopped; 2 cups dry red wine; 4 sprigs flat-leaf parsley; 2 sprigs thyme; 12 ounces small cipolline or pearl onions; Kosher salt; 3 tablespoons unsalted butter; 3 large red onions, thinly sliced; ⅓ cup bourbon; 2 cups low-sodium chicken broth; ½ cup pearl barley; Freshly ground black pepper; Prepared cornbread (for serving)