Preheat a grill pan over high heat for 3 to 4 minutes.

Combine the ground mustard, onion powder, garlic powder, salt, cayenne, and allspice in a small bowl.

Rub the flank steak all over with the dry rub mix.

Let the steak sit for 10 minutes to allow the flavors to meld.

Place the steak on the hot grill pan.

Cook for 5 minutes on each side for medium-rare.

Remove the steak from the grill pan and let it rest for 10 minutes before slicing.

While the steak rests, prepare the chimichurri sauce.

In a medium mixing bowl, combine the finely chopped parsley, finely chopped cilantro, and finely chopped capers.

Add the minced garlic to the bowl.

Pour in the red wine vinegar, salt, red pepper flakes, and ground black pepper.

Stir to combine the ingredients.

Add the olive oil to the bowl and mix well until everything is thoroughly combined.

Let the chimichurri sauce sit for 30 minutes so that the flavors can blend together.

Slice the flank steak against the grain.

Serve the sliced flank steak with the chimichurri sauce.