Pat the steaks dry. Rub with olive oil and season generously with salt and pepper.
In a small bowl, combine olive oil, vinegar (or lemon juice), parsley, garlic and chilli flakes. Season with salt and pepper and set aside.
Preheat the BBQ or a frying pan to high. Cook the steaks for 2–3 minutes per side for medium-rare (longer if preferred). Transfer to a board and allow to rest for 5 minutes.
Cut the ciabatta and spread with softened butter and toast on the BBQ or under the grill until lightly golden, rub with the cut side of the garlic.
Slice the steak thinly across the grain. Layer onto the toasted ciabatta, spoon over Chimichurri, and press the sandwich gently to hold everything in place.
Serve with pickle slaw or grilled corn on the cob.