Bring a large pot of heavily salted water to a rolling boil. Add the pasta and cook according to package directions until al dente. Before draining, reserve about 1/2 cup of the starchy pasta water. Drain the pasta and set aside.

While the pasta cooks, heat 1 tablespoon of olive oil in a large skillet or Dutch oven over medium heat. Add the diced onion and cook, stirring occasionally, until softened and translucent, about 5-7 minutes.

Add the minced garlic and red pepper flakes (if using) to the skillet. Cook for another 1 minute until fragrant, being careful not to burn the garlic.

Add the chopped kale to the skillet. It might seem like a lot, but it will wilt down significantly. Cook, stirring frequently, until the kale is tender and bright green, about 3-5 minutes.

Pour in the vegetable or chicken broth and bring to a simmer. Stir in the heavy cream and softened cream cheese. Whisk gently until the cream cheese is fully melted and incorporated into the sauce, creating a smooth, creamy base.

Remove the skillet from the heat. Stir in the lemon zest, lemon juice, and half of the freshly grated Parmesan cheese. Season with salt and freshly ground black pepper to taste.

Add the cooked and drained pasta to the skillet with the sauce and kale. Toss everything together, ensuring the pasta is well coated. If the sauce seems too thick, add a splash of the reserved pasta water, 1-2 tablespoons at a time, until it reaches your desired consistency.

Serve immediately, garnished with additional grated Parmesan cheese and a fresh grind of black pepper, if desired.