Peel the carrots, parsnips and sweet potato then cut them into 2-inch pieces of approximately the same size for even cooking.
Scrub the potatoes and if they are large, cut into pieces approximating the same size as the carrots, parsnips, and sweet potato.
Trim and peel the onions.
Put the vegetables in a large bowl and toss with the olive oil. Transfer to a baking sheet in a single layer. If they are too close together and heaped on each other they will end up 'steamed' rather than roasted.
Sprinkle generously with salt and black pepper.
Roast the vegetables at 425 degrees for 20-25 minutes. Check often and remove when they are fork-tender but still have a little resistance when pierced. Enjoy!
Notes
For added color include red cipollini onions or chunks of red onion
Divide the vegetables onto two sheet pans if necessary
Other options for the vegetables would be rutabagas, turnips, beets and fennel.
Check the vegetables after 15 minutes. If any of the vegetables appear to be done transfer them to a plate cover with foil and keep warm until all the vegetables are tender.