The night before, put the peas in a large bowl with the bicarbonate of soda and cover them with plenty of cold water, at least 5 cm above the peas. They roughly double in size as they soak.
Drain and rinse the soaked peas, then tip them into a pan with fresh water to cover. Bring to the boil and skim off any foam.
Simmer gently for 30 to 35 minutes until the peas are completely soft and starting to break down, topping up with hot water if the pan looks dry.
Beat the peas hard with a wooden spoon so they crush into a thick, rough mush that still holds a little texture rather than becoming a smooth paste.
Stir in the butter and mint, season generously with salt and pepper and loosen with a splash of the cooking liquor if needed. Serve hot with pie and chips.