In a large soup pot over medium-high heat melt butter, add onion, celery, carrot and fresh herbs to pot and cook for 5 minutes until veggies begin to soften. Add a pinch of salt, stir well.
Add mushrooms to pot and cook for an additional 2-3 minutes.
Add stock and turkey, stir well. Reduce heat to medium-low, and simmer.
Meanwhile: make the soup dumplings. In a small bowl whisk together eggs and milk, in a larger bowl whisk together flour, salt, and baking powder. Add eggs/milk to flour mixture and stir well with a wooden spoon, add fresh thyme and stir. Mixutre will be quite thick and sticky.Increase temperature of soup and bring to a boil. Dip two teaspoons into oil and form a small (1/2 the size of a golfball) ball of dough, drop directly into the soup. Repeat with the remaining dough. The dumplings are done when they have floated to the top and cooked for 3-5 minutes.
Ladle 1 cup of liquid from the soup into a large bowl, add sour cream and 3 tbsp flour, whisk until creamy and no lumps remain. Pour contents into soup pot and stir well to incorporate. Cook for an additional 3-5 minutes to help the soup thicken.
Taste to adjust seasoning (add additional salt and pepper if needed).
DUMPLING NOTES:Alternatively, you can cook the dumplings separately in boiling water and add a few to each bowl. When storing the soup you can store it with the dumplings inside the soup. If you have extra, leftover dumplings, saute in butter and onions for a real treat!