In a blender or food processor, blend half of the peas with the onion until you have a smooth purée. Add the eggs, flour, baking powder, mustard powder and salt, then blend until combined. Stir in the remaining peas.
Heat the butter and oil in a non-stick frying pack. Drop spoonfuls of the pea batter into the pan. Cook for 3 minutes, flip, then cook for a further 3 minutes. Remove from the pan and keep warm. Repeat with the remaining batter and serve the fritters straight away.
Cook’s tip: for brunch with a difference, create a tower effect using 2 pea fritters layered with rashers of crisp smoked bacon, a few rocket leaves and some slow-roasted tomato halves