Remove steaks from the refrigerator about 30 minutes before cooking.
Preheat a heavy-based frying pan, char-grill pan or barbecue over high heat until hot. Brush or rub steaks with olive oil then season with sea salt flakes and freshly ground black pepper. Cook steaks over high heat to your liking, following instructions below. Turn steaks once using tongs, not a fork. To test if the steak is ready, use the blunt end of the tongs or your finger to press the meat in the thickest part.
Cook on one side until moisture is pooling on top. Cook on second side until moisture is pooling on top. Reduce heat slightly; cook until steak feels very firm when pressed.
Cook on one side until moisture is pooling on top. Cook on second side until surface moisture is visible. Cook until steak feels springy when pressed with tongs.
Cook for a few minutes per side, depending on the thickness. Cook until steak feels very soft when pressed with tongs.
Remove steak from the heat and transfer to a warmed plate. Cover loosely with foil and rest for 5 minutes; this will ensure the steak is more juicy and tender. Serve steak with pan juices and wholegrain mustard.