Combine first 9 ingredients in a soup pot. Bring to a boil, then turn down to medium low, cover and simmer for 10 minutes.
Stir in peanut butter and milk. Return to a gentle simmer and cook uncovered for 5 minutes.
Stir in spinach and cook until wilted, 2 to 3 minutes. Remove from heat. Top with peanuts and green onion, if desired.
Refrigerate leftovers within 2 hours.