Place a large soup pot over medium heat and melt the butter. Transfer 1 tablespoon of the melted butter to a small bowl and set aside. Add the onion to the pan, season with salt and pepper, and cook for 4 to 5 minutes or until softened and lightly golden brown. Stir through the garlic and continue to cook for 30 seconds. Carefully pour in the broth and bring to a boil. Stir in the gravy, if using, along with the turkey and vegetables.
In a large bowl, stir together the flour, baking powder, and ¾ teaspoon of salt. Add in the buttermilk and the reserved melted butter as well as the chives and half of the parsley. Stir just to combine.
Stir the lemon juice and remaining parsley into the soup. Using two spoons or a small spring-loaded ice cream scoop, drop approximately 1-ounce (heaped tablespoon) dumplings into the pot leaving some space between each. Cover the pot with a lid, turn the heat down to low, and cook for 15 minutes or until the dumplings are cooked through.
Divide the spinach into serving bowls and spoon over the hot soup and dumplings to wilt. Serve immediately.
