Soak peas in three times their volume of water with bicarbonate of soda for 12 hours.
Drain and rinse the soaked peas under cold water.
Simmer peas in boiling water for 90 to 120 minutes until mushy.
Beat in 25g of butter and season with salt and pepper.
Blend egg yolks and mustard while gradually adding 300ml oil to create mayonnaise.
Stir in lemon zest and juice, then chill the mayonnaise in the fridge.
Whisk flour, yeast, sugar, salt, vinegar, and beer into a thick batter.
Let the batter ferment for 30 minutes until bubbling.
Heat oil to 190°C in a deep-fat fryer or heavy pan.
Coat fish in batter and fry for 4 to 6 minutes until golden brown.
Fry remaining batter droplets until crisp to create scraps.
Assemble by spreading peas on buttered baguette halves and topping with fish, mayonnaise, and scraps.