In a small bowl combine the butter and the flour, set aside. In a shallow, wide bottomed pan, place the potatoes, stock, saffron, and 1/2 teaspoon of salt. Bring to a boil and simmer for 1 minute. Add the halibut fillets and carrots. Cover and simmer for 4 minutes. When cooked, carefully remove it and add the spinach and green onions to the broth. While stirring with a spoon, add the flour butter mixture to the broth. The stock will thicken instantly, allow it to boil for 2 minutes. Taste and adjust the seasonings. Pour this sauce over the fish and arrange the vegetables.