Season the chicken breasts on both sides with salt and pepper and sprinkle lightly with cumin. Place the chicken in a shallow bowl and pour over the lemon juice. Cover and leave to marinate in the refrigerator for 1–2 hours.
Make up the gravy mix with 300ml of boiling water, or according to the packet instructions, and set aside.
Heat the oil and butter in a frying pan. Lift the chicken out of the marinade, reserving the marinade. Pat dry the chicken breasts. Cook the chicken over a moderate heat for 5 minutes until golden brown, then reduce the heat and cook gently for a further 5 minutes on each side. Transfer the chicken to a heated serving dish and cover.
Pour the reserved marinade into the frying pan and cook over a moderate heat for 1–2 minutes until reduced slightly. Add the onion gravy to the pan, bring to the boil, then reduce the heat and stir in the cream. Simmer the sauce for 2–3 minutes until thickened, then spoon it over the chicken. If you would like a garnish, use a vegetable peeler to remove strips of zest from the lemon and choose some attractive sprigs of sage.