Rub 15ml of olive oil onto the steak and season with salt and pepper.
Cook the steak in a hot griddle pan for 3 to 4 minutes.
Rest the meat for 2 minutes and cut it in half.
Rub the remaining 15ml of olive oil onto the bread and toast it on the griddle pan.
Toss the onion slices with vegetable oil, salt, and pepper in a bowl.
Griddle the onions for 5 to 6 minutes until golden and soft.
Reduce the wine in a small saucepan by half and stir in the cheese until melted.
Add the double cream to finish the sauce.
Assemble the sandwich by layering the steak and onions between the bread slices and drizzling with sauce.