Add mushrooms to the drippings in the pan. Cook over medium-high heat for 3 minutes, stirring occasionally. Whisk broth, mustard, cornstarch and the remaining ¼ teaspoon salt in a small bowl or measuring cup; add to the pan with the mushrooms. Cook, stirring, until thick and bubbly, about 1 to 2 minutes. Cook, stirring, for 1 minute more. Serve the steak and vegetables with the sauce.