Heat the oil in a large saucepan over medium-high heat.
Season the steak with 1/2 teaspoon salt and 1/4 teaspoon pepper and cook until browned, about 5 minutes. Transfer the steak to a bowl and set aside.
Add the mushrooms and onions to the pan and cook until the liquid has evaporated. Add the wine and simmer until reduced by half, 5 to 6 minutes. Stir in the soup and 1/4 cup water and bring to a boil. Add the steak and juices from the bowl and simmer, 2 minutes.
Divide into individual bowls and sprinkle with the parsley, if using.