Remove the stalks from the spinach and wash the leaves well.
Blanch the leaves for a few seconds in a large pan of boiling water until they are just wilted, then drain well and refresh in cold water to prevent further cooking. Drain again, squeezing out excess water.
Combine the steak with the shallots, garlic, one tablespoon of the fish sauce and some freshly ground black pepper, then set aside. (The soup can be prepared in advance up to this point.)
Just before you are ready to eat, bring the chicken stock to a simmer in a saucepan and season it with the remaining fish sauce, the lemon juice, sugar and chilli.
Add the blanched spinach and stir in the beef and its marinade. Bring the soup back to simmering point, add a few grindings of freshly ground black pepper to taste and serve at once.