
Preheat the oven to 350ºF. Grease a 9-by-13-inch baking dish with 1 tablespoon of the sun-dried tomato oil. Bring a large pot of salted water to a boil. Add the pasta and cook for 2 minutes less than the package directs. Reserve 1 cup cooking water, then drain.
Heat the remaining 1 tablespoon of sun-dried tomato oil in the pasta pot over medium-high heat. Add the sausage and cook, breaking up the meat with a wooden spoon, until golden brown, 6 to 8 minutes. Add the garlic and red pepper flakes and cook for 1 minute.
Add the flour to the pot and cook, stirring, until incorporated, about 30 seconds. Add the milk, reduce the heat to medium low and cook, stirring, until the mixture simmers and begins to thicken, about 4 minutes. Stir in 1 cup of the parmesan and cook until incorporated, about 30 seconds.
Add the sun-dried tomatoes and kale to the sauce and stir to coat. Add the reserved cooking water and stir to wilt the kale, about 2 minutes. Season with salt and pepper. Add the pasta and toss to combine.
Transfer the pasta to the prepared baking dish and spread evenly. Sprinkle with the mozzarella and the remaining 2 tablespoons parmesan. Bake until bubbly and golden, 25 to 30 minutes.