Turnip dauphinoise is gorgeous. I had it with puy lentils braised in garlic, rosemary & red wine, with green veg on the side.
They are good cubed and added to chunky stews, handful of pearl barley makes this extra nice I think.
Add to roasted wintery type veg, like carrots, onions, parsnips.
You can slice them and use them to top hotpot type things, like a lentilly shepherds pie, then sprinkle a bit of cheese on the turnips.
They make a good alternative to cubes of potato in sag aloo.
Can you guess I have a surplus too?! 
Apparently, not only are the leaves edible too, they are realy really good for you, lots of vitamin K. I am thinking of trying an all turnip sag aloo.... leaves and roots!
I have never tried mashing though, might do that soon.