This is a quick and light dinner, suffused with the sturdy flavor of fresh sage and potatoes. See the variation that follows on how to turn this into a summertime stew.
Finely chop the onion. Core, seed, and chop the pepper.
Heat the oil in a large saucepan or deep, large skillet over medium heat. Add the onion and bell pepper and stir-fry for a moment until you hear them sizzle. Reduce the heat to low, cover the pan, and simmer until tender, about 10 minutes.
Meanwhile rinse, stem, and thinly slice the mushroom caps