If you can't find demi-glace, don't worry, just follow recipe and you'll still get an incredible mushroom sauce to top steak. I like using crimini mushrooms (which are actually baby portobello mushrooms).
Ingredients:4 tablespoons butter
2 shallots, minced
1 pint of mushrooms, sliced
1 cup chicken broth
1/2 cup red wine
3 tablespoons Cognac
1 tablespoon demi-glace (optional)
salt and freshly ground black pepper
minced chives and parsley
Heat the butter in a frying pan over medium heat. Add the shallots and saute for 30 seconds. Add in the mushrooms, and saute for 2 minutes. Add in the broth, wine, cognac and demi-glace. Simmer until sauce is reduced in half and thickened, about 2 minutes. Season with salt and pepper (if using demi-glace, just season with pepper). Sprinkle with chives and pepper just before serving.