Bring a large pot of water to a boil and salt generously (about 4 quarts of water, 2 tablespoons salt). Add pasta and cook according to the package instructions. With 2 minutes left, add in your chopped up greens and boil for about 2 minutes. Reserve about 1 cup pasta water, then drain and set aside.
Wipe out the pot so the bottom is dry. Over medium-low heat, add oil, garlic, fennel seed, red pepper (if using), beans, and ½ teaspoon salt to the pot. Cook just until the garlic begins to soften, about 1-2 minutes, stirring occasionally.
Add the pasta and greens back to the pot along with the remaining ½ teaspoon salt and stir to coat in the spicy garlic oil. Add in about ½ cup pasta water and ½ cup cheese and stir constantly, until a glossy sauce forms. If it looks dry, add in a generous splash of that pasta water. As the pasta sits, it will soak up the liquid. Usually right before serving I’ll add in another generous splash of pasta water to loosen it up (or just use tap water).
Serve topped with more cheese, black pepper, and a squeeze of lemon, if using.