In a large pot or Dutch oven, melt the butter and saute the onions over medium-low heat for 10 to 15 minutes, until translucent. Add the flour and cook over low heat, stirring constantly, for 2 minutes. Add the chicken stock and bouillon cubes to the sauce. Bring to boil and simmer over low heat for 1 more minute, stirring, until thick. Add 2 teaspoons salt, 1/2 teaspoon pepper, and heavy cream. Add the cubed chicken, frozen vegetables, sherry, and parsley. Mix well.