Reduce the heat to medium. Add another tablespoon of oil if the skillet is dry, then add the onions. Cook, stirring occasionally, until softened, about 4 minutes. Sprinkle the flour over the onions and cook, stirring, 1 minute. Slowly pour in the milk while stirring constantly to dissolve the flour, then stir in the chicken broth. Increase the heat to medium high and bring to a simmer while stirring. Simmer until slightly thickened, about 2 minutes. Add the kale, artichokes and cayenne, then add the chicken and any collected juices. Cook, stirring, until the kale and artichokes are hot and the chicken is just cooked through, about 4 minutes. Stir in 3 tablespoons of the Parmesan.