Blend half the peas with the onion in a food processor until you have a smooth purée. Add the eggs, flour, baking powder, mustard powder and 1 tsp 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.