I peeled and quarted turnips, then roasted with carrots and onions in olive oil and maple syrup, but the turnips were really bitter. Is there a way to roast them (or even another cooking method) to remove that bitterness? How long should they be roasted?
Or do I simply need to pick a better batch of turnips?
Yeah, maybe you just had some bitter turnips. It sounds like you did all the right things to hide the bitterness.
If you want to try a different cooking method, I tend to like turnip boiled and mashed. If the turnip is old (and likely bitter) you can add an apple. I've also read that you can stir in baking soda after the turnips have boiled to remove the bitterness. You would then need to rinse thoroughly. I've never tried this however.
I don't think there is anything you can do to remove the bitterness, though as you attempted, you can mask it somewhat with sweetness and salt. And yes, some turnips are distinctly less bitter than others. Try to find some "baby" turnips, they tend to be milder.
I discovered that cooking them with tomatoes made them less bitter. Pondering this - I thought it might be the acid. So I cooked them about half done in VERY salty water with a couple of tablespoons of vinagar or lemon juice. Then I rinsed them well & finished cooking them in plain water. I used them in a 50/50 mash of cauliflower in place of mashed potatoes. Much better!
By the way I raise my own & have done so is several different locations with different types of soil, watered less, watered more, planted in shade, in full sun -- 'nuut'in hepped' as my Granny would say!