Just to clear a few things up first: when I say turnips I mean the ones that are white with a light purple tinge; the vegetable which is pale yellow with a purple top is a swede. Both are good, but the flavour of swede tends to be a bit stronger. Roasting the turnips brings on a slightly different flavour, more pronounced, while the sweetness of the apples and cider glaze add an almost sweet and sour aspect.
A little tip when choosing turnips: give them a good squeeze. They need to be hard as a rock; any softness means they are likely to be woody inside.

800g turnips – it seems a lot but you will lose quite a bit in the peeling
sunflower oil
sea salt
2 Granny Smith apples
50g granulated sugar, plus a sprinkling to season
100ml cider, preferably dry
2 tbsp cider vinegar
Preheat the oven to 190°C/fan 170°C/gas mark 5.
Turnips first: slice off the tops and bottoms and cut into quarters so you end up with wedges – they are easier to peel like this than when they are whole. Using a small sharp knife, peel off the skins. Pop the wedges into a roasting tray, drizzle with a little oil and season with salt. Use your hands to mix them up so that they are all coated. Place in the oven; they will take around 30 minutes to cook depending on size. You will know when they are ready as they will be nice and soft when you give them a little prod and they will have some golden colour to them. Give them a turn now and again.
While the turnips cook, prepare the apples. Stand them up and cut them in half, then remove the core and cut each half into five wedges; this will help the dish to look pretty, as you are mimicking the shape of the turnips. Put the apple wedges in a bowl, give them a glug of oil and season with salt and a good sprinkle of sugar.
Into a frying pan (using a frying pan helps the glaze to reduce quicker, due to the surface area) pour the cider, vinegar and 50g of sugar. Bring to the boil, then simmer for 5 minutes. Transfer to a bowl, but don’t clean the pan.
Five minutes before the turnips are finished, take the frying pan, give it a quick wipe, then put it over a high heat with no oil. When it is very hot, add the apple wedges and flash-fry on both sides to caramelise them; this will take a few seconds, so be careful, and they may stick. Turn the heat off and leave the apples to finish cooking in the remaining heat.
Meanwhile remove the turnips from the oven, transfer to a serving bowl, add the apple wedges, then drizzle with the cider glaze. Or add the apples to the roasting tray with the turnips, drizzle over the glaze and mix carefully. Serve.
If you like the apple and turnip mix you can create another similar dish by peeling and chopping the turnips, peeling, coring and chopping the apples, then boiling them together. Drain well, then mash with a spot of butter and seasoning.