Combine onion, sausage, and celery in a large saucepan. Cook until sausage has no pink remaining. Drain any fat.
Add potato, seasoning, one can of cannellini beans, and 3 cups chicken broth. Bring to a boil, reduce heat and simmer 10 minutes.
Meanwhile, combine remaining cannellini beans and 1 cup chicken broth in a blender. Blend until smooth.
Add to soup and simmer an additional 5 minutes.
Remove from heat, stir in pesto. Season with salt and pepper to taste.
Garnish with parmesan cheese and parsley if desired.
