In a pan sweat the onion in 11 tablespoons of the butter until it’s soft, sweet and clear. Put to one side. Smear the remaining butter over a deep ovenproof frying pan, into which you place the first layer of sliced turnip. Add a smattering of your buttery onion mixture, season, follow with another layer of turnip, and repeat this process until the frying pan is full (the turnips will shrink during cooking, do not be disappointed).