Heat oil in a large pot over medium-high heat. Add onion and celery; cook, stirring often, until the onion is translucent, about 5 minutes. Add garlic, Italian seasoning and crushed red pepper; cook, stirring occasionally, until aromatic, about 2 minutes. Stir in beans, Parmesan rind (if using), broth, salt, pepper and bay leaf; bring to a boil over high heat. Reduce heat to medium-low and cook, stirring occasionally, until the vegetables are tender, about 20 minutes. Remove and discard Parmesan rind (if using) and bay leaf.

Pour 2 cups of the soup into a blender. Secure the lid on the blender and remove the center piece to allow steam to escape. Place a clean towel over the opening. Process until smooth, about 1 minute. (Use caution when blending hot liquids. Alternatively, pour 2 cups of the soup into a medium bowl; process until smooth using an immersion blender.)

Pour the pureed soup back into the remaining soup in the pot; stir to combine. Stir in lemon zest and lemon juice. Divide the soup among 4 bowls; sprinkle with Parmesan and garnish with parsley, if desired.