Preheat the oven to 425 degrees.
In a roasting pan, toss the onion, sweet potato, carrot, parsnip, celery root (and/or butternut squash), and beet with two tablespoons of the olive oil.
Season the vegetables with salt and pepper and roast for about 45 minutes, stirring once or twice, or until tender and lightly browned in spots.
Meanwhile, spread the walnuts in a nonstick skillet and toast over medium heat until fragrant and lightly toasted, about 6 minutes.
Once the walnuts have cooled, coarsely chop and set them aside.
In a large bowl, whisk together the lemon juice, balsamic vinegar, mustard, and the remaining tablespoon of olive oil and fold in the parsley.
Season with salt and pepper.
Add the vegetables and the walnuts to the dressing and toss.