In a large saucepan, heat the margarine over a medium heat, add the onion and cook until softened (about 5 minutes). Shred the spinach, add to the pan and stir until it wilts. Add the peas and stir again. Gradually add half of the stock, stirring, and bring to the boil. Simmer for 5 minutes, stirring occasionally. Puree the soup in the blender. Return to the pan and add the chopped mint and stir for a further 2 minutes, then add the rest of the stock and heat it through. Remove the pan from the heat, stir in the soy cream and reheat soup gently. Serve garnished with fresh mint leaves and some croutons.