Start your rice. See blog notes for cooking guidance. Once it's cooked, set aside.
Preheat your oven to 350F
In a medium hot pan, quickly sear the steak.
Remove from the heat and cut into bite-size pieces (at this point it will still be rare - it will continue to cook throughout the prep and in the oven). Set aside.
In the same pan, over medium heat add your thinly sliced onions. Let them soften and caramelize. Add in your halved jalapeños peppers if you are using them. Once the onions start to caramelize add in your brown sugar and let it cook for about a minute as you prepare your slurry.
In a small bowl, whisk together your soy sauce and corn starch. This is called a slurry and it will make the sauce thicker.
Add your slurry to your pan and stir to combine all the ingredients. Add in your cut up steak and bring up to a boil until it starts to thicken. Remove from the heat. If your sauce becomes too thick, you can add water until you reach the consistency you want. Water will not change the flavor of your sauce. Do not add in more soy sauce - that will change the overall flavor of the dish. Add in red pepper flakes (optional). Remove the jalapeños peppers.
Set aside a 3-4 tablespoons of sauce - to serve on the side.
In a large bowl, combine the onion steak mixture, the remainder of the sauce and cooked rice together.
Cut the tops off your pepper and remove the inside seeds. Place standing up in a casserole dish. Fill each pepper up with the steak and rice filling.
Bake in the oven at 350F for 45 minutes, until your bell peppers are tender.