In a Dutch oven, melt butter over medium heat. Add celery, mushrooms, onions and garlic; cook for 2 minutes.
Add turkey, peas, carrots and broth. Whisk in turkey gravy mix. Bring to a boil over medium-high heat.
Meanwhile, make dumplings. In a medium-size bowl, combine baking mix, sour cream, sage and black pepper, stirring until soft dough forms. When soup comes to a boil, drop heaping tablespoons of the dumpling dough into soup.
Maintain a low boil and cook, uncovered, for 10 minutes. Cover and cook for 10 minutes more. Dumplings are done when they are dry in the center. Ladle soup and dumplings into soup bowls and serve hot.