Preheat the oven to 200°C, gas mark 4.
Wash the spinach, then wilt down in a saucepan (no need to add any extra water).
Plunge the wilted spinach into cold water and drain through a colander. Squeeze out as much water as possible with your hands, scrunching the leaves up into a ball. Slice finely and put to one side.
Separate the eggs, placing the yolks into a small bowl and the white into a large mixing bowl. Whisk the whites until you have stiff peaks, then fold in the spinach and ham.
Line a baking tray with parchment and spoon the white mixture onto the tray to form four cloud shapes. Using a teaspoon make a small indentation in each, then carefully place one yolks into each cloud.
Place into the oven and bake for 5-7 minutes, or until the white is tinged golden but the yolk is still runny.
Sprinkle with the walnuts and chives to serve.