To make the teriyaki sauce, whisk all the sauce ingredients in a small sauce pan and simmer on low for 3 minutes, until the sauce has thickened slightly then remove from heat.
Add one tablespoon of the cooking oil to a pan over medium-high. Season steak with salt and pepper then sear the steak 2 minutes on each side and remove from pan.
Add the remaining 1/2 tablespoon of cooking oil. Add the mushrooms and sauté until mushrooms are lightly browned, 2 minutes, and then add the peppers and cook until crisp tender, another 2 minutes.
Place the steak back into the pan, reduce heat to low and add the teriyaki sauce, let simmer for 1 minute. Serve the teriyaki sauce, mushrooms and peppers on top of the steak.