Heat the butter and olive oil over medium heat in a large skillet.
Add the mushrooms and cook until wilted and toasted. Add the shallot and cook until wilted. Add the garlic and stir until fragrant, about 30 seconds.
Carefully add the cognac a little at a time. Have a pan lid nearby in case it flames.
Cook until reduced by half.
Add the stock, Worcestershire, and Dijon mustard. Stir to mix well. Cook until the sauce reduces and thickens. Pull a wooden utensil through the sauce. If the separated sauce is thick and doesn't flow together quickly, it is ready for the cream. Add it and stir to mix. Cook until the sauce thickens slightly. Serve over steaks. If desired, season with salt and pepper to taste.
Notes
Make it Gluten-Free
Make this sauce gluten-free by using a gluten-free Worcestershire sauce, such as Lea & Perrins*. Also, check the ingredients label on the stock or broth to ensure it's gluten-free.*Note that Lea & Perrins is gluten-free in the U.S., but not in Canada or other countries.