Bring a large pot of water to a boil. Add the kale and cook until tender, about 2 minutes. Drain, squeeze out any moisture and dry very well.
Heat 2 tablespoons of the oil in a large nonstick skillet over medium-high heat. Season the chicken with salt and pepper, then add it to the skillet and cook, stirring occasionally, until golden on some sides and most of the pink is no longer visible, about 4 minutes. Transfer to a bowl with a slotted spoon (the chicken will not be fully cooked).
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.
Preheat the broiler to high. Spray an 8-inch broiler-proof baking dish with cooking spray and spoon the mixture inside. Top with the panko and remaining tablespoon Parmesan. Broil about 3 inches from the element until golden brown in spots, about 3 minutes.