Sauté aromatics: Warm olive oil in a skillet over medium heat. Add garlic and red pepper flakes (if using). Cook for 1–2 minutes until fragrant and lightly golden.
Simmer beans: Stir in cannellini beans and broth. Season with salt and pepper. Simmer gently for 5–7 minutes until beans are warmed through and broth has slightly reduced.
Finish with flavor: Add lemon zest and juice, then stir in Parmesan until melted and creamy.
Taste and serve: Adjust seasoning as needed. Garnish with fresh parsley and extra Parmesan, if desired.