Fill a large bowl with ice water and set it aside.Set a large stock pot full of salted water to boil over high heat. Add the kale ribbons to the boiling water and blanch until slightly tender, about 3-4 minutes. Scoop the kale from the water and toss it into the ice bath to stop it cooking and set its color. Let cool, then drain the kale in a large colander. Squeeze any excess water out of the kale and set it aside.Melt the butter in a large skillet over medium heat and add the minced onion. Cook until translucent, then add the garlic and red pepper flakes and cook another 2 minutes. Add the kale and the cream and turn the heat to medium-high. Cook, stirring occasionally, until the cream thickens to a sauce consistency, about 5 minutes. Season to taste with salt and pepper and remove from the heat.Serve as is, or process in a food processor to achieve the texture you desire.