Drain the spinach and squeeze out the liquid. Separate the eggs, mix the yolks, sour cream and parsley until smooth. Season with salt, pepper and nutmeg to taste and mix in the grated Emmentale.
Beat the egg whites until very stiff and then fold into the egg yolk mixture. Heat the oil in an ovenproof skillet, pour half of the egg mixture into it and distribute by gently tilting the pan. Fold the spinach into the remaining egg mixture and immediately add to the pan. Bake in the oven at 200°C (approximately 400°F) for about 20 minutes. Cover with aluminum foil part way through baking if it begins to brown too much.
Slide the baked frittata onto a plate, cut into pieces and serve.