Chicken in Truffle Cream Sauce

Name: Chicken in Truffle Cream Sauce
Ingredients: 1 (4–5-lb.) chicken, cut into 8 pieces; 4 white onions, peeled and quartered; 2 bottles dry white wine; Salt; 1 bouquet garni (4 sprigs parsley, 2 sprigs thyme, and 1 bay leaf, tied with kitchen twine); 3 medium waxy potatoes, peeled and quartered; 3 medium carrots, peeled, trimmed, and halved lengthwise; 3 medium leeks, white part only, washed, trimmed, and halved lengthwise; 12 pearl onions, peeled; 2 tbsp. butter; 2 tbsp. flour; 1 ⁄ 2 cup heavy cream; Leaves from a bunch parsley, finely chopped; 1 medium fresh black truffle, cleaned; Pinch freshly grated nutmeg; 6 sprigs fresh rosemary