Roasting celery root brings out a sweetness that helps balance some of its trademark earthy bite. The longer you cook it the sweeter it gets. Be wary when using older, cellared celery root, as they tend to be drier, thus requiring longer cooking times than fresher ones in season or at a farmer's market. If you have pomegranate seeds available, a small handful would add some crunch and color to this side dish.
- Adjust an oven rack to the middle position and put a baking sheet inside. 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. Toss the cubes in the bowl with the cumin seeds, olive oil, 1 teaspoon salt and a few grinds of black pepper.
- Halve the red onion and thinly slice. Remove the hot baking sheet from the oven and spread the cubes 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 and put back into the oven to roast. (If the onions are in contact with the pan for the entire cooking time, they could get too dark or burn.)