Aargh, root vegetables
You could also dice and roast, they do cook about the same time. But the tastes are so different, I question whether combining just the two is fair to either.
To me, rutabaga goes better with potato, sweet potato, carrot, parsnips, and other starchy vegetables. While, turnip goes better with lighter vegetables like celery, celery root, brussels sprouts, fennel bulb, and so forth.
I got this recipe from the General board a long time ago, and I apologise that I don't remember who posted it - especially because it has become such an old favourite:
Dice evenly and combine equal parts of:
Toss with olive oil, salt/pepper, and roast at 400F until browned and lovely.
You might play around with the proportions, and substitute rutabaga for parsnip. But the carrot and fennel bulb were a very nice counterpart to the other root vegetables. Or maybe add a sweet yellow beet into the mix?