Cut pork fillets lengthwise to the middle. Rub inside and outside with salt and pepper. Cover inside with sage leaves. Mix feta cheese with oil and fill the fillets with it.
Tie together with kitchen string. Grill for about 15-20 minutes, turning several times. For the vinaigrette, peel the onion. Dice olives and onion finely. Wash and chop the herbs. Mix vinegar, salt and pepper.
Gradually beat the oil into it. Stir in olives, onions and herbs. Serve with the fillet. Baked potatoes (about 1 hour grilling time) taste good with it.