"A sworn lover of parsnips, this is but one more way to fix them. The use of celery root, a.k.a. celeriac, was an accident since I thought I was buying a rutabaga but it was an excellent addition. I've served this to company with rave reviews and it's fairly healthy if you use the oil sparingly. Great way to enjoy fall and winter produce."
Chop parsnips, carrots, celery root, rutabaga, and yellow onion into 1-inch pieces and place in a large sealable container. Add garlic, rosemary, olive oil, salt, and pepper; seal container and shake well to coat vegetables evenly. Distribute vegetables in a single layer in two 9x13-inch baking dishes; scrape remaining oil and seasonings from container onto vegetables.
Roast in preheated oven until browned and easily pierced with a fork, about 45 minutes.