Sweet peppers: slice 2 or 3 bell peppers into long strips and then cut the strips in half.
Add a teaspoon of olive oil to a medium-sized skillet over medium heat. When the oil is shimmering, add the bell pepper pieces and then toss them around in the oil. Add a pinch of salt and ground black pepper and cook the peppers for around 5 to 7 minutes or until the peppers start to soften a little.
Once the peppers have softened, remove them to a sealed container and place them in the refrigerator until it's time to make the sandwich.
Roll toasting: if you desire a toasted roll, heat a large skillet or griddle over medium heat. Open the roll like a book or if the rolls are split all the way through, lay them down in the pan to toast the interior. Toast for 2 to 3 minutes or until the bread is just starting to turn a bit brown. Remove the roll to a cooling rack for filling later. Add any condiments as soon as the roll cools a little.
Chicken cheese steak cooking: get all your chicken cheese steak ingredients sliced and ready because cooking happens fairly quickly.
Remove the stem, clean, and prepare the portabella mushroom for cooking. Slice the portabella mushroom into long strips around a quarter of an inch wide and then slice those lengthwise into smaller pieces.
Add a tablespoon of olive oil to the large skillet or griddle over medium heat. When the oil starts to shimmer, add the portabella mushroom pieces, the thinly sliced onions, and a spoonful of your sweet peppers. Season the vegetables with a pinch of salt and ground black pepper. Cook for one minute and then stir the vegetables together.
Move the vegetables to one side of the pan or griddle and then add the thinly sliced chicken to the other side of the pan. Season the chicken with a good pinch of salt and ground black pepper.
Cook the chicken, stirring often (keep stirring the vegetables too) until there is no visible pink on the meat, about 5 or 6 minutes.
Combine the vegetables and chicken into one pile if you're using a 12-inch roll or two piles if you're using two 6-inch rolls. Top each pile with slices of cheese.
Cook for another 1 to 2 minutes or until the cheese is melting and then with two spatulas or a large spoon move your piles of meat, vegetables, and cheese into the roll.
Serve and enjoy.