Based on a recipe from Bon Appétit
Heat oil in a large Dutch oven or heavy pot over medium heat. Add lemon, anchovies, and garlic. Cook, stirring occasionally, until lemon is softened and brown in spots and anchovies fall apart, about 5 minutes.
Add broccoli rabe; season with salt and pepper and cook, tossing occasionally, until bright green and crisp-tender, about 5 minutes.
Add beans and ½ cup water to pot. Bring to a boil; reduce heat and simmer, stirring occasionally, until flavors have melded and liquid is reduced by half (you still want it to be saucy), about 5 minutes. Mix in parsley and 2 tablespoons grated Parmesan.
Transfer to a platter, sprinkle with red pepper flakes, if desired, and top with additional grated Parmesan.
If you like your broccoli rabe on the crunchy side, remove from the mix before adding the white beans. Keep it on the side while the beans simmer, then add it back to the pot just before the beans are done.