Place the chicken breasts in a single layer in a non-metallic dish. Pour over 3 tablespoons of the oil, and the lemon juice. Scatter over the sage leaves, turn the chicken so that the breasts are evenly coated, then cover and leave to marinate for about 30 minutes.
Lift the chicken breasts from the marinade and reserve the sage leaves separately. Pat the breasts dry. Strain the marinade into a small bowl.
Heat the butter and the remaining oil in a frying pan, add the chicken, and cook for about 10 minutes over a moderate heat until browned. Turn the chicken breasts over, season with salt and pepper, and tuck the sage leaves around them. Cook for a further 10 minutes until the underside is brown and the chicken is cooked through. Transfer the chicken to a warmed serving plate, cover and keep warm.
Tilt the pan and pour off the fat. Place the pan back on the heat and stir in the reserved marinade, scraping up any brown bits from the bottom of the pan. Boil until reduced to a brown glaze. Serve the chicken in slices, on a bed of puy lentils, if liked. Pour the remaining marinade over the chicken and garnish with the sage leaves.