Heat oil in a 12-inch skillet over medium-high heat. Season both sides of each steak with salt and pepper. Cook 2 steaks at a time for 1 1/2 minutes per side for medium-rare. (Do not flip steaks more than once.) Add more oil if needed. Transfer to a plate. Cover, and let rest at least 5 minutes.
Add mushrooms to drippings in skillet, and cook over medium-high heat, stirring occasionally, until tender, about 4 minutes. Add stock and juices from plate to skillet. Bring to a boil, scraping up brown bits. Stir in mustard, and simmer until sauce lightly coats the back of a spoon, about 3 minutes. Pour sauce over steaks, and sprinkle with herbs.