Adjust an oven rack to the middle position and put a baking sheet on it. Preheat the oven to 425 degrees F.
Lightly chop the cumin seeds and add to a medium bowl. Trim the ends off the celery root, then peel and discard the exterior. Cut the root into 3/4-inch-thick planks and then into 3/4-inch cubes. Add the cubes to the bowl with the cumin seeds, then add the olive oil, 1 teaspoon salt and a few grinds of black pepper and toss.
Carefully remove the hot baking sheet from the oven and spread the celery root cubes on it in an even layer. Add the onion slices to the bowl the celery root was in and coat with the residual oil and seasonings. Arrange the onions on top of the celery root (if the onions are in contact with the pan for the entire cooking time, they could get too dark or burn).
Roast until the cubes begin to brown on the bottom, about 15 minutes, then toss the vegetables around, scattering the onions around the pan, and flatten again into 1 layer. Continue to roast until the vegetables are very soft when pierced with a knife, about 10 minutes more. Allow to cool for a few minutes before tossing with the parsley leaves and a drizzle of olive oil.