Heat olive oil in a large soup pot over medium heat.
Add sliced sausage and cook until browned.
Remove excess fat if needed, then add chopped onion.
Sauté onion until soft and translucent.
Stir in minced garlic and cook until fragrant.
Add carrots and celery and cook for 5 minutes.
Stir in white beans, chicken broth, and Italian seasoning.
Season with salt and black pepper.
Bring soup to a boil, then reduce heat and simmer for 20 minutes.
Add spinach and cook until wilted.
Stir in lemon juice if using and remove from heat.