For a slightly sharper flavor, substitute two pounds of turnips for the rutabaga. If using turnips, you may need less milk -- add it gradually; stop when the mixture is fairly smooth.
Peel the rutabaga, and cut it into 1-inch chunks. Place in a medium saucepan with enough water to cover, and bring to a boil over medium-high heat. Reduce to a simmer, and cook until tender, about 25 minutes. Drain, and set aside.
Cut the stems off the kale, and discard. Roughly chop the kale, and wash in a large colander. Coat a large saute pan with olive-oil cooking spray, and place over medium-high heat. Add the onion and garlic, and saute until translucent. Add kale in batches, allowing each to wilt before adding more. Cook until all the kale is wilted, add salt and pepper, and transfer to the bowl of a food processor. Puree mixture until the kale is smooth; add reserved rutabaga. With machine running, add milk gradually; stop when mixture is smooth, and serve.