First make the sauce: in a small pan bring to the boil the pomegranate & lemon juice with sugar and reduce it by half to a thin syrup. Add the balsamic vinegar and reduce it further to a thicker sauce. When it starts to caramelise, i.e. it's becoming sticky, turn the heat off, cover and let it rest.
Heat a griddle pan or grill/BBQ. Season the steaks then rub them with little oil. Cook them on each side for 30 seconds then keep turning them every minute until it's cooked to your liking (about 3-4 minutes on each side for medium).
Transfer the steaks onto a plate. Cover with foil and rest for five minutes.
Add the mustard to the pomegranate sauce - if it's too thick add a spoonful of balsamic vinegar to it and a drop of lemon juice. Taste it and adjust the sweetness/sharpness to your liking.
Scatter the salad leaves onto a serving dish or plates.
Cut the meat diagonally into thin slices. Arrange these on top of the salad leaves, grind some pepper on top. Scatter the pomegranate seeds over the salad and drizzle with the sauce.