In a large (4-quart) saucepan, warm the oil over medium-low heat. Add the onions, and cook, stirring occasionally, until they are translucent but still firm. Add the red pepper flakes and garlic and the kale, and stir until the kale is fully wilted. Add stock to cover by about ½ inch. Bring to a simmer. Cover, and continue to simmer until the kale is tender but not mushy, about 30 minutes. Add salt as needed.