Combine diced onion, Italian sausage, and chopped celery in a large saucepan. Cook over medium heat until sausage has no pink remaining, about 8-10 minutes. Drain any excess fat.

Add chopped potato, Italian seasoning, one can of cannellini beans, and 3 cups chicken broth. Bring to a boil, then reduce heat and simmer for 10 minutes, or until potatoes are tender.

Meanwhile, combine the remaining cannellini beans and 1 cup chicken broth in a blender. Blend until completely smooth.

Stir the blended bean mixture into the soup and simmer for an additional 5 minutes to thicken.

Remove the saucepan from heat, then stir in 1 tablespoon of pesto. Season with salt and freshly ground black pepper to taste.

Garnish individual servings with 1 tablespoon of shredded Parmesan cheese and chopped fresh parsley if desired.