Lamb Stew from Stewed

Name: Lamb Stew from 'Stewed'
Ingredients: 3 tablespoons olive oil, plus 3 if discarded after searing the lamb; 2 pounds boneless lamb leg, cut into 1-inch chunks; Kosher salt and cracked black pepper; Flour, for dusting; 1 stick unsalted butter, plus 2 tablespoons; 1 cup carrots, scrubbed, 1/4-inch dice; 2 cups Spanish onions, 1/4-inch dice; 1/2 cup celery root, peeled, 1/4-inch dice; 1 teaspoon chopped fresh rosemary; 1 teaspoon chopped fresh sage; 2 cups ruby port wine; 5 cups beef stock; 1 cup stewed tomatoes; 1/2 cup turnips, scrubbed, 1/4-inch dice; 1/2 cup parsnips, scrubbed, 1/4-inch dice; 1 bay leaf; 1/2 cup red grapes, thinly sliced; Garnish; Chopped fresh parsley; Lemon zest