Make the chimichurri by combining the cilantro, parsley, green onion, garlic, lemon zest, salt, olive oil, and white wine vinegar, in a medium bowl. Mix to combine. Take away ⅓ cup of the chimichurri from the bowl.
Place the Flank Steak in a dish which is similar in size to the piece of meat. Pour the ⅓ cup of the chimichurri on the steak. Flip the meat over to make sure there is chimichurri on both side. Cover the dish and let it marinate at room temperature for 1 hour.
Heat the BBQ grill to 400F. Take the steak out of the marinade, letting any excess oil drip off of it before placing it on the grill. Discard the marinating liquids.
Cook for 3-4 minutes per side for an internal temperature of 135F. Take the grilled steak off the heat and onto a plate. Cover right away with aluminum foil and let the meat rest for 10 minutes
While the steak is resting, toast the hoagie rolls and spread some of the Dijon mustard on the bottom part of each roll.
Slice the steak in ⅛ inch slices. Divide up the slices of steak evenly and stack over the 4 toasted hoagie rolls.
Drizzle about 1 tablespoon of the remaining chimichurri over the steak slices in each sandwich. Close up the sandwich with the top part of the roll and enjoy!