Melt the butter in a large saucepan over medium heat, and cook the onions until tender.
Mix in the potatoes, celery, carrots, chicken, and garlic.
Season with Italian seasoning.
Pour in milk, gradually stir in cornstarch, and bring to a boil.
Mix in tomatoes, broth, and tomato paste.
Return to boil, reduce heat to low, and simmer 1 hour.
Season with salt and pepper.