Preheat the barbecue while you prepare the steak.
Carefully slice the flat iron steak horizontally, leaving about 1 to ½ inches uncut so it opens like a book. Cover it with plastic wrap.
Use a meat tenderizing mallet to flatten the steak to about ¼ inch thickness. Season both sides with salt and pepper to taste.
Spread 1 tbsp. of tapenade on the open steak, pressing it into the beef. Add a handful of arugula leaves on top.
Roll up the beef to create a roulade, making sure the fillings are enclosed. Secure with twine or toothpicks.
Place the roulade on the grill and cook until it reaches your desired doneness.
While the meat cooks, prepare the Charmoula sauce. Begin by juicing half a lemon. Next, in a food processor, blend lemon juice, garlic, herbs, cinnamon, cumin, cayenne pepper, allspice, and ¼ cup of olive oil. Season with salt and set aside.
Pat dry capers with a paper towel. In a small saucepan, heat the remaining olive oil. Fry the capers until crisp, about 1-2 minutes. Remove them from the oil and set aside on a paper towel-lined plate.
After cooking, let the meat rest for approximately 5 minutes. Slice the roulade into rounds of your desired thickness.
To plate the steak, spoon a dollop of Charmoula sauce onto each plate. Arrange the sliced roulade over the sauce.
Add a handful of arugula leaves and sprinkle fried capers liberally. For extra flavour, use a cheese curler to shave Iberico or Manchego cheese over the arugula leaves and drizzle quality olive oil over the dish.