Peel the mushrooms and remove the stalks, then grill them top-side up
Place the couscous, olive oil, cumin, cayenne pepper and lemon rind in a bowl. Season, then stir the flavourings through the couscous
Bring the chicken stock to the boil and stir it into the couscous. Cover and leave for 5 minutes, then fluff the grains with a fork. Stir in the tomato, lemon juice, parsley and peppers. Fill each mushroom with some of the couscous mixture and pack down firmly. Grill until the couscous is golden. Serve hot or cold.