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).
Cover with tinfoil (shiny side down; the matte side encourages the transference of heat through the foil whereas the shiny side will deflect it slightly, which at other times is just what you want). Place into a medium to hot oven for approximately 1 hour so it is thoroughly cooked. Check for a giving center with a knife. When satisfied take out of the oven, remove the foil and allow it to calm down for about 5 minutes.
Now to serve, tip it upside down onto the appropriate-sized plate (being careful not to burn yourself in this maneuver). You should now have a brown and yielding turnip bake (it will not be crisp), calling out to be eaten with some roast lamb.