Bring the water to a simmer. Add the onion, potatoes, salt, pepper, garlic, and crushed red pepper, if desired. Simmer for 10 minutes, until the potatoes are soft. Blend the ingredients with an immersion blender (or in a standard blender and then return to the pot). Return the soup to a simmer. Add the kale to the simmering soup, cooking it for about 5 minutes. Remove from the heat, then immediately stir in cooked chorizo or portugese sausage.