Combine the olive oil, Worcestershire sauce, red wine vinegar, parsley, salt and pepper in a small bowl. Place the flank steak in a large zipper sealable plastic bag and pour the marinade over the top. Massage the steak and marinade together, seal the bag and refrigerate for at least 2 hours or as long as overnight.

To make the Béarnaise sauce: Combine half the bunch of tarragon, the shallots, peppercorns, vinegar and wine in a small saucepan and bring to a boil. Lower the temperature to a simmer and simmer until the liquid has reduced by half. Let the mixture cool and strain it through a fine strainer.

Combine the tarragon reduction, egg yolks, and salt in a blender jar. Blend until well mixed. Slowly drizzle in the melted butter.

Season with salt and cayenne pepper. Add the leaves from the remaining half bunch of tarragon and buzz in the blender a couple of times. Hold the Béarnaise sauce in a bowl covered with plastic wrap, over a barely simmering water bath.

Pre-heat an outdoor grill or grill pan. Grill the marinated flank steak for about 8 to 10 minutes on each side until an instant read thermometer inserted into the center of the steak registers140º F. Transfer the steak to a cutting board and let it rest for at least 5 minutes.

While the steak is cooking, prepare the onions and bread. Pre-heat a large frying pan. Add olive oil and sauté the onions until lightly browned, about 5 minutes, seasoning with salt and freshly ground black pepper. Deglaze the pan with Worcestershire sauce and toss to coat the onions. Remove the onions from the pan and cover with foil to keep them warm. Wipe out the pan and add another tablespoon of olive oil. Toast the bread slices in the olive oil on both sides until golden brown.

Thinly slice the flank steak on the bias against the grain of the meat. To assemble the sandwiches, place a slice of bread on a serving plate. Top the bread with some arugula and slices of steak. Cover the steak with fried red onions and pour warm Béarnaise sauce on top. Serve immediately.