Pre-heat grill to high. Season steaks with salt and pepper, drizzle with oil. Place on the grill for 2-3 minutes each side or until caramelised and cooked to your liking. Rest for half the amount of cooking time.
Butter bread and chargrill.
Assemble sandwich, butter the base of the bread with aioli, top with Mesculan, followed by tomato, then steak, caramelised onions, and placed bread on top.