Heat a large skillet or pan over medium heat. Add the butter and let it melt.
Add the chopped onion and cook until it becomes translucent, about 2-3 minutes.
Add the sliced mushrooms to the pan and cook them for a few minutes until they start to brown.
Stir in the minced garlic and thyme sprigs, cooking for another 1-2 minutes until fragrant.
Sprinkle the flour over the mushroom mixture and stir well to combine.
Pour in the white wine, stirring constantly, and allow it to simmer for a few minutes until it has reduced by half.
Slowly add the chicken stock while stirring continuously to avoid lumps. Bring the mixture to a gentle simmer.
Once the sauce has thickened slightly, add the cream and black pepper. Stir to combine, and let it simmer for another 5 minutes to allow the flavours to meld. Taste and season with salt as needed.
Add the leftover turkey to the pan and cook for 5-7 minutes, or until the turkey is heated through and the sauce has thickened to your desired consistency.
Remove the thyme sprigs and discard them.
Serve the leftover turkey fricassee with cooked rice, mashed potatoes, or pasta. Garnish with fresh chopped parsley for a burst of color and flavor.