Wash kale and working in two batches, stack the kale leaves and slice in 1/2 inch strips. Set aside Add 2 cups of stock, the chopped onion and olive oil to blender. Blend for a minute or two, or until it is completely smooth. Add that to the crock pot and add the remaining 4 cups of broth, kale, shredded chicken, zest of all three lemons and the 2 tlbs of fresh lemon juice. Add a pinch of salt. Let cook for 6 hours on low crock pot setting, stirring once or twice.