Whisk together the sauce. Place the steak and half of the sauce in a bag. Let marinate for at least an hour or all day! When you're ready to prepare the sandwich, heat oil in a pan over medium heat. Add the onions and cook until caramelized and golden brown. Remove from the pan and set aside. Cook the steak strips in batches, 3-4 minutes per side. When the steak strips are all brown, add back in the browned steak, remaining sauce from the marinade, and sauteed onions. Cook for an additional 5-7 minutes. Serve in a baguette.
