Place the butter beans into a large container and cover with several inches of warm water; let stand 1 hour. Drain.
Fill a large pot with lightly salted water and bring to the boil over high heat. Add the ham hocks and return to the boil; cook at a boil for 1 minute. Drain and rinse.
Combine the butter beans and ham hocks in a stockpot; add the ginger, tomato, onion, green chillies, tamarind base and 700ml water. Season with salt to taste. Bring the mixture to the boil; reduce heat to medium-low and simmer until the pork is tender and the butter beans are soft and bursting from their skins, about 20 minutes. Add the spinach to the soup and cook until the spinach has just wilted.
Tamarind soup base
Available online or in Asian shops. A popular brand is Knorr.