Blanch the spinach in lightly salted water for 2 minutes. Then immediately quench in ice water and allow to drain.
Fry the onion and garlic in the olive oil, add the spinach. Pour in the stock, add the potatoes and simmer for about 15 minutes until the potatoes are cooked through. Season to taste with salt, pepper, saffron and a little nutmeg. Puree everything.