Using the smallest blade on a spiralizer, cut the carrots, parsnips and celery root. Transfer to a medium bowl and cover with ice water. If using a purple carrot, cut it last and keep it in a separate bowl to avoid its color from bleeding. Let the vegetables sit in the water for at least 15 minutes to firm. Drain the vegetables and dry well with a salad spinner.
In a large bowl, whisk together the olive oil and lemon juice. Add the vegetables and herbs, and toss to coat. Season with salt and pepper.
In a small bowl, season the Greek yogurt with salt and pepper. Using the back of a large spoon, smear the hummus on one side of a platter. Pile the salad high in the center, then garnish with a dollop of the seasoned Greek yogurt, the pumpkin seeds, pistachios, mint and parsley, and serve.