Make chimichurri: In a small bowl, mix parsley, oregano, garlic, vinegar, olive oil, chilli flakes, salt, and pepper. Stir and set aside to let flavours develop.
Cook steak: Season steak with salt and pepper. Heat a grill or frying pan over high heat with a little olive oil. Cook steak for 2–3 minutes per side (for medium), or to your liking. Rest for 5 minutes, then slice thinly against the grain.
Assemble panini: Layer bottom half of roll with provolone, sliced steak, roasted capsicum, rocket, and a spoonful of chimichurri. Close with top half.
Toast: Cook in a sandwich press or hot pan (weighted with a second pan) for 2–3 minutes until bread is golden and cheese melted. Serve immediately.