Melt the butter in a large pan and soften the onions, but do not allow them to colour. Add the peas to the pan with the stock. Bring to the boil and simmer for about 5 minutes for frozen peas, but for up to 15 minutes for fresh peas, until they are cooked. Be careful not to overcook fresh peas or they will lose their flavour.
Remove from the heat and purée in a blender or food processor. Add the yogurt or cream and grate in a little nutmeg. Reheat gently if necessary, and serve sprinkled with chives.