Make the chimichurri first. On a large chopping board (secured by placing a damp kitchen cloth underneath it), finely chop the parsley and mint. Transfer to a jar and add the remaining ingredients. Check for seasoning and balance of flavours and adjust if necessary. Leave to stand until needed.
Meanwhile, season the lamb steaks and cook for 3–4 minutes on each side in a large pan over medium-high heat with the olive oil. Remove and set aside to rest for about 5 minutes before serving immediately with the chimichurri.