Cover the peas with cold water and bring to the boil.
Peel and halve the onion.
Drain and drain the peas and place in a saucepan with the onion, chicken stock, butter, salt and pepper.
Cover and cook until the peas are tender and have absorbed most of the liquid.
Mash the peas with the liquid.
Meanwhile, peel potatoes and cut into thin slices. Boil in salted water for about 20 minutes, drain and steam until all the liquid has evaporated and puree as well.
Put half of it in a large bowl, save the other half.
Then add the mashed peas to the mashed potatoes, whisking vigorously. If the mash is not yet firm enough, fold in the remaining mashed potatoes by the tablespoonful until it is the right consistency.
Season the mashed peas with salt and pepper and keep warm until ready to serve.