Combine oil, garlic, mustard and seasoning in a bowl. Place steak in a gastronorm, pour over marinade. Leave in the cool room to marinate for at least 1 hour.
Bring steaks to room temperature before cooking.
Remove steaks from marinade and cook on a hot grill for 2-3 minutes each side or to an internal temperature of 49°C. Leave to rest for half the cooking time, lightly covered in foil.
Slice thinly and serve with chimichurri sauce and grilled vegetables.