I tossed big chuncks of carrot, parsnip and onion in olive oil...how hot should the oven be?
It's my understanding you can roast vegetables slow up to 325°F, or a fast as high as 500°F. The oven temperature will depend on the type of vegetable. Slow roasting works especially well with carrots or other root vegetables. Many vegetables are easily adapted to a range of temperatures--experiment.
For hard veg, I like to start out at 350 for 15-20 minutes and then crank the oven up to 475 to get them to caramelize a bit on the outside. Just be very careful to keep an eye on them at that temp (I set the fire alarm off...twice...last time...).
I tend toward 425 to 450 for most vegetables.
425 for carrots and parsnips, but maybe 400 for the onions if they are sliced because they might burn.