Heat a large sauté pan on medium heat. Add 1 tablespoon of butter. Once the butter is melted, add the onion and cook, stirring, until transparent, about 4 minutes. Remove with a slotted spoon and set aside.
In the same pan, melt the remaining butter. Increase the heat to medium-high and add the mushrooms, cook, stirring occasionally, for about 4 minutes, until the moisture is released from the mushrooms and they begin to brown. Sprinkle with the nutmeg, tarragon and thyme, season with pepper.
Add the gravy to the mushrooms and continue to cook and stir for another minute. Reduce the heat to low and add the sour cream to the mushrooms. You may want to add a few tablespoons of water to thin the sauce. Mix in the sour cream thoroughly. Do not let the sauce come to a simmer or boil or it will curdle.
Stir in the cooked white and dark meat turkey and reserved onions. Season with salt and pepper to taste. Serve immediately over noodles. Garnish with chopped parsley.