In a small bowl, mix together the pepper, rosemary, mustard, garlic and salt. Pat the filets dry and then season on both sides with the rub.
Heat the oil over medium high heat in a medium nonstick or cast-iron skillet. Add the beef filets and sear until golden brown on both sides, about 3 minutes per side.
Transfer to a baking sheet and bake until an instant read thermometer reads 125 F for rare (or to your desired cooking temperature).
While the steaks finish in the oven, add the port, beef and rosemary to the pan the steaks were cooked in, and raise the heat to medium high. Scrape up any beef bits from the pan and cook until well reduced to a light syrup.
Remove from the heat and stir in the balsamic, the Green & Blacks Organic chocolate, butter and salt and pepper to taste.
Remove the steaks from the oven, and transfer to serving plates. Drizzle with the port chocolate sauce and garnish with a pinch of fleur de sel and fresh rosemary or parsley.