Place the ham bone, onion and 3 qts. water in an 8-qt. soup kettle or Dutch oven. Bring to a boil. Reduce heat; cover and simmer for 2-1/2 hours. Remove ham bone. When cool enough to handle, remove meat from the bone; set aside. Discard bone.
Add wild rice to stock. Simmer for 1 hour or until tender. Remove from the heat; drain and reserve stock. Set rice and onion aside. Add enough water to stock to make 2 qts.; set aside.
In the same kettle, melt butter. Stir in flour until smooth. Gradually stir in reserved stock. Bring to a boil; cook and stir for 3 minutes or until slightly thickened. Remove from the heat.
In a small bowl, combine cream and yolks. Add 1 cup stock to egg mixture, then stir into kettle. Return soup to heat; heat gently over medium-low. (Do not boil.) Add reserved rice, onion and ham along with mushrooms, chicken broth, pepper, thyme and parsley. Thin with milk if necessary. Yield: 12-16 servings (4 quarts).