Steam or boil the spinach until soft. Add the peas and cook for 4-5 minutes. Drain thoroughly in a colander, pressing the spinach to get rid of excess liquid. Put the spinach and peas into a food processor with the butter and whizz to a purée. Season with salt and black pepper. Put the mixture into a wide ovenproof serving bowl, cover loosely with foil and keep warm in a very low oven.