Poach the chicken pieces in boiling water for 3-5 minutes. Remove from water once cooked and drain on kitchen paper.
Melt the butter is a saucepan and sauté the onion until soft but not coloured.
Stir in frozen peas, pour in chicken stock, bring to boil and cook for 3 minutes.
Stir in the couscous, remove from heat and cover, leave for 6 minutes until the couscous cooks.
Fluff the couscous with a fork and add the diced chicken, serve hot or cold.