Wash the spinach carefully, snipping off any hard stems. Drain. Chop the leaves and stew them in their own juice in a large covered saucepan until tender, about 15 minutes. If using frozen spinach, defrost it in a colander and simmer until cooked. Allow to cool.

Beat the yogurt and garlic together, and add the mixture to the pan. Mix well and season to taste with salt and pepper.