Bring a large pot of salted water to a boil. Add the kale and cook until tender, about 2 minutes. Drain, squeeze out any moisture and dry very well.
Heat 3 tablespoons olive oil in a large nonstick skillet over medium-high heat. Season the chicken generously with salt and black pepper, then add to the skillet and cook, stirring occasionally, until golden on some sides and most of the pink is no longer visible, about 4 minutes (the chicken will not be fully cooked). Transfer to a bowl using a slotted spoon.
Reduce the heat to medium. Add another tablespoon oil if the skillet is dry, then add the onion; season with a pinch each of salt and black pepper. Cook, stirring occasionally, until the onion is softened, about 4 minutes. Sprinkle the flour over the onion and cook, stirring, 1 minute. Slowly pour in the milk, 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. Cook until slightly thickened, about 2 minutes. Season with another pinch each of salt and black pepper.
Add the kale, artichokes and cayenne to the sauce, 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 1/4 cup parmesan. Taste and adjust the seasoning if necessary.
Preheat the broiler. Coat a 3-quart broiler-proof baking dish with cooking spray and add the casserole mixture. Top with the panko and remaining 1/4 cup parmesan. Broil until golden brown in spots, about 3 minutes.