Caramelized onions:
Add olive oil to a heated wide, thick-bottomed frying pan. Spread the onions out and caramelize till soft and golden brown on medium heat, stir regularly. Set aside.
Preheat the pan on high heat.
Drizzle oil on both sides of the steaks and season with black pepper and salt.
Sear the steaks on high heat till golden brown outside but medium rare (or as you like it) inside. Set aside.
Brown butter in the pan, and add mozzarella cheese and smoked cheddar cheese. Stir till it melts.
Toast the baguettes. Slice the steaks
To assemble the sandwich, slice the baguettes and fill with sliced steak, onions and spinach leaves. Top with melted cheese and serve.
