Put the split green peas in a large pot with the baking soda and enough water to cover the peas four times over. Cover and allow to rehydrate in the fridge overnight.
Drain the peas in a colander and rinse with cold water to remove any lingering baking soda.
Return the peas to the pan and cover with cold water by 1/2-inch. Add some salt, bring to a boil and then reduce the heat to a simmer. During the simmering, discard any pea foam that forms on top using a slotted spoon. The peas are done cooking once they just lose their bite, 30 to 40 minutes. Overcooking results in soup, which isn't what we're going for.
Remove from the heat, add the butter and stir to fully incorporate. Taste and add more salt if required.
Serve immediately with fish and chips, or cool and store for up to 3 days.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.
Recipe courtesy Grant MacNaughton, Mac's Fish and Chip Shop