Wash and clean the turkey liver thoroughly and cut into small pieces. Peel the onions and cut into rings. Wash the apples, peel them if necessary, and cut them into ½ cm thick slices.
Heat butter or margarine in a large pan over medium-high heat. Fry the onion rings for 4-5 minutes until golden brown, then remove and drain on kitchen paper.
In the same pan, fry the apple slices for 4-5 minutes until golden, then remove and drain on paper.
Fry the turkey liver pieces in the remaining fat for 3-5 minutes on each side over medium-high heat, seasoning with salt and pepper, until fully cooked (internal temperature 74°C / 165°F).
Pour the white wine into the pan and reduce on medium-high heat for 2-3 minutes, scraping up any browned bits.
Return the liver, onions, and apples to the pan.
Sprinkle the sauce thickener over the mixture and stir on medium heat for 1-2 minutes until the sauce thickens.