Bistro-style slow-roasted duck

Start to finish: 4 hours 50 minutes (50 minutes active)

Servings: 4

5 1/2-to 6-pound duck

Kosher salt and ground black pepper

1 tablespoon vegetable oil

1 small yellow onion, finely chopped

1 small carrot, coarsely chopped

2 cloves garlic, smashed with the side of a knife

1 tablespoon tomato paste

1 cup dry red wine

1 stalk celery, chopped

2 sprigs fresh thyme

1 bay leaf

3 cups low-sodium chicken broth

2 tablespoons all-purpose flour

2 tablespoons drained bottled green peppercorns, packed in brine

1 tablespoon Dijon mustard

Heat the oven to 250 F.