This recipe makes enough broth for the Tortellini in Capon Broth. Charring the onions and leeks over a flame imparts a richer flavor to the liquid.
Place onions and leeks directly over a stovetop flame; cook, turning, until slightly charred, about 5 minutes. Transfer to a large stockpot. Add capon, celery, carrots, tomatoes, basil, parsley, chicken bouillon, and salt; cover with 6 quarts water. Place over high heat; bring to a boil. Reduce heat to medium low; simmer partly covered until capon is cooked through, about 1 1/2 hours.
Carefully lift the capon from the broth, and transfer to a cutting board. Cover capon with a clean wet cloth, cover with aluminum foil, and set aside in a warm place. Pass the broth through a cheesecloth-lined sieve into a large container. Using a wooden spoon, press down to squeeze out as much liquid as possible from vegetables, and discard solids. Using a large spoon, remove as much fat that rises to the top as possible. Transfer stock to a large saucepan; return to high heat.
One hour before serving, bring a large pot of water to a boil. Pierce the sausage all over with a fork. Wrap in cheesecloth, and tie ends with kitchen twine. Add the prepared sausage, reduce heat to medium, and cook for 1 hour. Remove from heat; keep warm. When ready to serve, slice the sausage and capon into serving pieces and arrange in a serving dish with Mixed Vegetables.