Preheat the oven to 425 degrees F. Coarsely grind the coriander and cumin seeds in a spice grinder (or crush with the bottom of a skillet). Combine the parsnips, turnips, celery root, olive oil, garlic, ground spices, paprika, 1 1/2 teaspoons kosher salt and a few grinds of pepper in a large bowl; toss to coat. Divide the vegetables between 2 rimmed baking sheets and spread in an even layer. Roast until lightly browned around the edges, about 30 minutes.
Sprinkle the vegetables with the pepitas and gently stir. Continue to roast until the vegetables are tender and the pepitas are lightly toasted, about 15 more minutes. Transfer to a serving dish. Sprinkle with sea salt and cilantro.