Preheat oven to 450°.
Remove giblets and neck from duckling; reserve for another use. Rinse duckling under cold water; pat dry. Trim excess fat. Starting at neck cavity, loosen skin from breast and drumsticks by inserting fingers, and gently pushing hand between skin and meat.
Combine 1/2 teaspoon salt, 1/2 teaspoon pepper, coriander, and allspice. Sprinkle spice mixture under loosened skin. Tie ends of legs together with cord. Lift wing tips up and over back; tuck under duckling.
Spread rock salt evenly in bottom of a shallow roasting pan. Place duckling, breast side up, on rock salt. Pierce skin several times with a meat fork. Insert meat thermometer into meaty part of thigh, making sure not to touch bone. Bake at 450° for 50 minutes or until thermometer reaches 180°. Cover duckling loosely with foil; let stand 10 minutes. Discard skin before serving.
Prepare cabbage mixture while duckling roasts. Soak chestnuts in a bowl of water for 30 minutes. Drain. Cut a slit in the shell on the rounded side of the chestnut. (Make sure slit goes the whole way through shell. If not, they will explode.) Arrange chestnuts in a single layer on a microwave-safe plate. Microwave at high 2 minutes. Cool 5 minutes;peel. Chop chestnuts; set aside.
Heat oil in a medium saucepan over medium heat. Add onion; sauté 3 minutes. Add cabbage, apple, sugar, vinegar, and bay leaf; cover, reduce heat to low, and cook 5 minutes. Stir in chestnuts, broth, juniper berries, 1 teaspoon salt, and 1/4 teaspoon pepper; cook, uncovered, 10 minutes, stirring occasionally. Discard bay leaf. Serve with duckling.