For the marinade:
1) Combine the mustard and balsamic vinegar in a small bowl. Add the garlic and drizzle in olive oil while stirring. Season with salt and pepper, to taste.
For the cheese steak:
1) Put the steak in a resealable plastic bag. Pour in 3/4 of the marinade and seal the bag. (reserve the remaining marinade for the bread.) Let the steak marinate at room temperature for at least 30 minutes.
2) Put the peppers and onions on the griddle. Coat the onions with bbq sauce. Once the peppers and onions are charred, keep warm.
3) Remove the steak from the marinade and put on the griddle. After 5 minutes turn the steak over to sear the other side, about 12 minutes total cooking time, for medium-rare. Transfer the steak to a cutting board and let rest for 10 minutes before slicing.
4) Cut the bread lengthwise and toast on the griddle.
5) Brush the remaining reserved marinade on both sides of the bread.
6) Arrange the steak on the bottom half of the bread, then cover with the peppers and onions. Top with the cheese and cover with the other half of the bread. Cut into 4 pieces and serve.
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