This one is almost easier to make than it is to eat since the entire thing will have a tendency to slide apart when you sink your teeth into it. But delicious none the less.
Halve the bread and put olive oil and salt on it. Then add some baby salad leafs and a sliced avocado and sprinkle with some more olive oil, S&P and a generous dash of the balsamic vinegar.
Fry the steak according to your liking (rare or bust) and then paste it with plenty of dijon mustard. Finally slice the steak into thin strips that you put on the sandwich and top with the mustard sprouts.