Remove any sinew from the calf's liver. Wash, dry and roughly chop the thyme. Cover the calf's liver slices in olive oil, thyme and pepper. Marinate in the refrigerator for about two hours.
To grill, spray the barbecue rack with rapeseed oil. Remove the calf's liver slices from the oil, remove the remaining herbs and grill the liver on both sides for about two minutes. Add salt only after grilling and serve immediately.