1
Bring a medium pot of salted water to a boil. Wash and dry produce.
Halve, peel, and thinly slice onion. Halve, core, and thinly slice bell pepper into strips. Using the tip of a spoon, remove and discard black gills from undersides of mushrooms; trim any large stems if necessary. Thinly slice mushrooms into strips (if strips are too long, cut them in half crosswise).