1. Preheat the oven to 400F. Peel the potatoes and cook in lightly salted boiling water for minutes, until very tender. Drain and mash with the egg yolks.
2. Stir in half of the Gruyere cheese and all of the flour. Season to taste with salt and ground black pepper.
3. Finely chop the spinach and fold into the potato mixture.
4. Whip the egg whites until they form soft peaks. Fold a little of the egg white into the mixture to slacken it slightly. Using a large spoon, fold the remaining egg white into the mixture.
5. Grease 4 large ramekins. Pour the mixture into the dishes; place on a baking sheet. Bake for 20 minutes. Remove from the oven and let cool.
6. Turn the souffles out onto a baking sheet and sprinkle on the remaining cheese. Bake again for 5 minutes; serve with salad greens.
VARIATION: For a different flavoring try replacing the Gruyere with a crumbled blue cheese, such as stilton or shropshire Blue, which have a stronger taste.