Season the steaks with salt and pepper. Melt the butter in a medium skillet or frying pan over a medium-high heat. Cook the steaks for about 5 minutes each side for medium-rare. Set aside for 5 minutes.
2) Meanwhile, heat a large nonstick pan or griddle over a medium heat. Using a serrated knife, slice the croissants in half lengthwise. In 2 batches place the croissant halves, cut side down, in the pan and cook for 30 seconds to 1 minute until lightly toasted.
3) Arrange the slices of Brie over one half of each toasted croissant. Add half the rocket leaves on top.
4) Slice the steak across the grain into 1/2 to 1-cm thick slices and add to each sandwich. Arrange the red pepper slices on top of the steak. Top with the remaining rocket and add the top half of the croissant. Serve immediately.