In a large saute pan, heat the olive oil over medium heat. Add the onions and saute until translucent, about 3 minutes. Add the garlic and kale, season with salt and pepper, to taste, and cook until wilted. Set aside to cool.
Once the mixture has cooled, add to a food processor and process until very smooth. Add a little water, if necessary, to help make a smooth mixture.
In another large pot, bring 5 cups of water and 2 cups heavy cream to a boil over medium heat. Whisk in the polenta and stir constantly until the polenta is creamy, about 20 minutes. If the mixture becomes too thick, add a little more water or cream.
Stir in the butter and season with salt and pepper, to taste. Fold in the kale and the Parmesan and adjust the seasoning, if necessary. Turn out into a serving bowl and serve.