Sauté onion and mushrooms in butter in a heavy skillet over medium heat for 1 minute, stirring frequently. Add steaks; cook 1 minute on each side.
Heat brandy in a small pan until just warm. Pour over steaks; ignite with a long match. When flames die down, remove steaks; set aside.
Add mustard, salt, pepper, and Worcestershire sauce to mushroom mixture; blend well. Return steaks to skillet; add wine. Simmer over low heat 5 minutes or until desired degree of doneness. Remove steaks to a warm serving plate; pour wine sauce over steaks. Serve immediately.