Chef John's Steak Diane
You can substitute top sirloin steaks for the tenderloin steaks.
You can substitute 2 cups rich, low-sodium or salt-free chicken broth for the 1/2 cup demi-glace. It will take longer to reduce.
You don't need to flame the cognac as the alcohol will evaporate as it cooks on the stove. But be careful--it can still flare up if the heat's high enough when you're reducing the sauce.