Heat butter in a large skillet over medium-high heat.
Fry the turkey thigh for 4-5 minutes per side until browned, then transfer to a roasting pan.
In the same pan, fry the onions and garlic for 2-3 minutes until softened.
Transfer the onions and garlic to the roasting pan with the turkey thigh. Add roughly 1 cup of water.
In a bowl, mix the ketchup, crème fraîche, and Cremefine together. Pour this mixture over the turkey leg and add the prunes.
Cover the roasting pan with foil and braise in a preheated oven at 160°C (320°F) for 90 minutes, basting the turkey leg with the braising liquid every 30 minutes.
Check that the turkey leg has reached an internal temperature of 74°C (165°F) at the thickest part. Serve with rosemary potatoes.