Soak peas in water for at least 1.5 hours or overnight.
Drain peas, place in a pan with fresh water and bicarbonate of soda.
Simmer covered for 1.5 to 2 hours until soft and mushy.
Beat in butter and season with salt and pepper.
Whisk flour, yeast, salt, sugar, beer, and vinegar into a thick batter.
Ferment batter for 30 minutes until bubbling.
Heat oil in a fryer or heavy pan to 190°C.
Coat onion rings in batter and fry for 2-3 minutes until golden.
Coat hake in batter and fry individually for 4-6 minutes until golden.
Drizzle remaining batter into oil to create crispy scraps.
Butter bread slices and cut in half.
Whisk egg yolks, mustard, and vinegar together.
Slowly whisk in rapeseed oil to form a thick mayonnaise.
Fold in capers, gherkins, shallot, herbs, salt, pepper, and lemon juice.
Serve hake with mushy peas, tartare sauce, bread, and lemon wedges.