Bring a large pot of generously salted water to a boil. Cook fusilli until al dente according to package directions. Reserve 1 cup of pasta water, then drain the pasta.

While the pasta cooks, heat olive oil in a large deep skillet or Dutch oven over medium-high heat. Add sausage and break it into crumbles with a wooden spoon. Cook until deeply browned and cooked through (internal temperature reaches 160°F/71°C for ground pork sausage), about 6–8 minutes. Transfer to a plate with a slotted spoon, leaving the drippings in the skillet.

Reduce heat to medium. Add diced onion to the same skillet and sauté until softened, about 4 minutes. Add garlic, sun-dried tomatoes, and red pepper flakes (if using) and cook 1 minute until fragrant.

Pour in chicken broth and heavy cream, stirring to combine. Bring to a gentle simmer and let it bubble for 3–4 minutes until slightly thickened.

Add chopped kale in handfuls, stirring until wilted, about 3 minutes. Return the browned sausage to the skillet and stir to incorporate.

Add the drained fusilli to the skillet along with 1/2 cup of the reserved pasta water. Toss everything together over low heat for 1–2 minutes, allowing the pasta to absorb the sauce. Add more pasta water as needed for a silky consistency. Remove from heat and stir in the grated Parmesan until melted and creamy. Season generously with black pepper and salt to taste.

Divide into bowls, top with torn basil, extra Parmesan, and a drizzle of olive oil if desired. Serve immediately.