Heat oven to 400°F. Grease a 4-quart baking dish. Bring a large saucepan of heavily salted water to a boil over high heat. Add 1/2 pound of pasta and cook 3 minutes short of al dente, drain pasta.
Heat 3 tablespoons oil in a large frying pan over medium-high heat. Once shimmering, add 1/2 diced onion and season with salt and pepper. Cook until translucent then add 4 cups kale, season again and cook until kale is slightly wilted.
Lower heat to medium-low and add cream, measured 3/4 teaspoon salt, 1/2 teaspoon sage, 1/2 teaspoon pepper flakes, and measured 1/4 teaspoon black pepper. Bring to a simmer, then stir in 4 ounces shredded Fontina and 2 ounces (half) of the Parmesan. Keep stirring until cheese is melted then turn off the heat.
Stir in cooked pasta, 1 1 /2 cups shredded chicken and 1 cup frozen peas. Toss until well coated. Taste to see if it needs more salt and pepper then pour into the prepared baking dish.
Combine 1/2 cup panko and remaining 2 ounces shredded Parmesan in a medium bowl and sprinkle this mixture over the top of the casserole.
Bake, uncovered, until top is golden and bubbling, about 30-35 minutes. Let rest 10 minutes, then enjoy!