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Preheat the broiler. Spray a 9-by-13-inch ceramic or metal baking dish with nonstick cooking spray.
Cook the pasta in a large pot of salted, boiling water until about 2 minutes less than package instructions. Reserve 1 1/2 cups pasta cooking water. Drain and set aside.
Meanwhile, heat a large Dutch oven or heavy saucepan over medium-high heat. Heat the olive oil in the pan, then add the onion and cook, stirring occasionally, until softened and starting to brown at the edges, 4 to 5 minutes. Add the garlic and red pepper flakes and cook until the garlic is very aromatic, another minute. Add the sausage and cook, crumbling into small chunks with the back of a wooden spoon, until browned all over, 4 to 5 minutes. Add 1 cup pasta water to the pan and scrape up any of the browned bits from the bottom of the pan. Pour in the heavy cream and cook, stirring frequently, until it comes to a simmer. Stir in the kale, then cover the pan and cook, stirring occasionally, until the kale is wilted and tender and the heavy cream has thickened, another 4 to 5 minutes. Sprinkle the nutmeg and about 1 teaspoon each of salt and pepper on the kale (taste here, as your sausage might have different seasonings). Remove from the heat and stir in the ricotta until just combined, followed by 1/4 cup each of the fontina and mozzarella. Add the pasta and stir until it is coated in sauce and the cheese starts to melt, adding pasta water if the mixture seems too thick. Taste and season with a little more salt, pepper and red pepper flakes if desired.
Transfer the mixture to the prepared baking dish and top it with the remaining 1/4 cup fontina, 1/4 cup mozzarella and the Parmesan. Broil the pasta until the cheese is brown and bubbling, about 5 minutes. Serve immediately.