For the steak: In a medium bowl, whisk together the hoisin sauce and vinegar. Add the flank steak and marinate for 30 minutes.
For the spicy slaw: Mix the mayonnaise and Sriracha together in a small bowl. Combine the carrots, cabbage and red onions in a large bowl. Add the spicy mayonnaise and combine thoroughly. Season with salt and pepper. Chill until using.
For the toasted baguettes: Heat a pan over medium-low heat. Butter the baguette halves, add them to the pan and toast until golden brown. Remove the bread from the pan, but leave the pan on the heat.
Add the flank steak to the pan and cook for 3 to 4 minutes, adding a splash of water to the pan if the sauce is too thick.
Divide the steak among the baguette bottoms, top with some spicy slaw and the baguette tops and cut in half. Enjoy!