Preheat the oven to 400°. In a medium baking dish, combine the parsnips with the butter, water and cumin and season with salt and pepper. Cover with foil and roast for 25 minutes, until just tender. Uncover and roast for about 25 minutes longer, until the water has evaporated and the parsnips are browned in spots. Transfer the parsnips to a bowl and garnish with the scallion.