How to Make It
Preheat the oven to 425°. In a medium roasting pan, toss the onion, sweet potato, carrot, parsnip, celery root and beet with 2 tablespoons of the olive oil. Season the vegetables with salt and pepper and roast for about 45 minutes, stirring once or twice, until tender and lightly browned in spots.
Meanwhile, spread the walnuts in a pie plate and toast until golden, about 6 minutes. Transfer the walnuts to a work surface and coarsely chop.
In a large bowl, whisk the vinegar with the lemon juice, mustard and the remaining 1 tablespoon of olive oil and fold in the parsley. Season with salt and pepper. Add the vegetables and walnuts to the dressing and toss. Top the salad with the feta and serve warm or at room temperature.