Heat the olive oil in a large pot over medium heat. Add the onion, celery, and carrot with a pinch of salt and cook about 5 minutes, until the onion softens. Stir in the garlic, rosemary, and red pepper flakes and cook 30 seconds.

Add the butter beans and broth. Bring to a boil, then reduce to a simmer and cook 15 minutes.

Remove from heat. Use an immersion blender to puree about half the soup, leaving some beans whole. Return to a gentle simmer.

Stir in the lemon juice, season with salt and pepper. Ladle into bowls and top with Parmesan and a drizzle of olive oil.