Roast the vegetables:
Heat the oven to 425°F. In a large bowl, toss the oil, garlic, onion, carrots, potatoes, sweet potato, celery root, salt, and pepper to coat the vegetables well. Spread them out in one layer on rimmed baking sheets and roast, stirring frequently, until tender and brown, 25 to 30 minutes.
Make the salad and dressing:
Tip:When I “plate” warm salads, I find that using tongs is the best way to evenly distribute the greens and other ingredients (the ones that tend to fall to the bottom of the bowl) over the serving plates. Or I reserve some of the garnishes to sprinkle on the plated salad.
When the vegetables are almost finished, put the greens in a large stainless-steel mixing bowl; set aside. (If you don’t have a stainless bowl, see below.) In a small bowl, whisk the herbs, vinegar, and olive oil. Season with salt and pepper. Drizzle half the dressing (reserve the rest) over the greens and set the bowl over a burner heated to medium. With tongs, toss the greens in the bowl over the heat just until the greens begin to wilt.
(If you don’t have a stainless bowl, heat half the dressing, reserving the rest, in a small saucepan and toss it with the greens in a mixing bowl off the heat.)
Add the hot cooked vegetables to the greens and toss well. Distribute the salad among four plates, top with the Parmesan, and drizzle on the remaining dressing or pass it at the table. Serve warm.