Melt the butter and warm olive oil together in a large skillet or Dutch oven set over medium heat. Add the leeks, onion, and rosemary or thyme. Cook for 3-5 minutes, stirring occasionally, until the vegetables begin to soften.
Add garlic and red pepper flakes. Cook for 30-60 seconds more, just until fragrant.
Stir in the greens and cook for 1-2 minutes, just until wilted.
Stir in the beans, broth, Parmesan rind, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Increase heat to bring the liquid to a boil, then reduce heat to low and simmer for 6-8 minutes, until reduced and slightly thickened. (If you want the dish to be more creamy, mash some of the beans against the side of the pan with a wooden spoon as they simmer.)
Remove pan from the heat, remove and discard the Parmesan rind, then stir in the lemon juice. Sample and season with more salt and pepper to taste. Divide into bowls and top with grated cheese, lemon zest, parsley, and more cracked black or red pepper, if desired. Enjoy!