Soak the peas in a large bowl, in three times their volume of water with the bicarbonate of soda for at least four hours or, if you have the time, overnight.
Drain the peas, rinse under the tap, and place on the stove in a large pan and cover with the water. Cover and bring to the boil and once boiled, reduce the heat and simmer the peas for 1½-2 hours, stirring from time to time.
The peas should be soft and mushy in texture but not too dry. If they are wet, continue cooking over the heat with the lid off to dry out a little. Beat in the butter and season.
Heat the oil for deep-frying in a deep heavy-based frying pan until a breadcrumb sizzles and turns brown when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Place the egg yolks and flour into a bowl and whisk to combine to a thick batter.
Whisk the egg whites in a separate bowl until soft peaks form when the whisk is removed.
Fold this into the batter, alternating with the sparkling water to make a light fluffy batter.
Dip the sea bream through the batter and fry for 3-4 minutes, or until golden-brown. (You may need to do this in batches.) Remove from the pan with a slotted spoon and set aside to drain on kitchen paper.
To serve, place the fish onto the plate, butter the bread and set alongside. Finish with a spoonful of mushy peas and wedge of lemon.