Prepare all of the ingredients before you begin cooking.
Mix the sauce and set it aside. You will need to grate tsp of fresh ginger. I use a zesting tool, but if you don't have one, you can use the small holes on a box grater. If you have no tools, dice the two teaspoons as finely as possible.
Stir the brown sugar, soy sauce, and water together. Add the mustard and mix until combined. Add the ginger, stir, and set aside.
Prepare the red pepper and onion by slicing into strips. Make sure to remove the seeds from the pepper. Set the veggies aside.
Slice the steak into thin strips. I used flat iron steak, but that is rather expensive, so flank steak is fine. You can even use a cheaper cut of meat if necessary.
Heat the olive oil in a frying pan. Add the steak.
Cook the steak, turning the slices occasionally, until cooked through, about four minutes.
Remove the steak to a plate and keep warm.
Add the veggies to the frying pan. If the pan is dry, add a bit more olive oil.
Cook the veggies until soft, about 4-5 minutes.
Once the veggies are cooked, return the steak to the pan.
Pour the sauce over the entire mixture and simmer at a low heat for at least five minutes. Don't let the sauce evaporate.
I love serving this over a bowl of cooked jasmine rice.