Yes, you can. I'm assuming you mean the instant "demi" where you mix it with water and get like 1 to 2 cups of sauce? There's different kinds so it depends which one you use. Is it the instant sauce type or the concentrated demi glace Gold type? If it's the instant sauce type, then mix it with water then scald it. In a seperate sauce pan, saute the mushrooms, deglaze or don't deglaze with red wine (if you deglaze it, let the wine reduce to au sec), then add the scaled demi and the peppercorns. Simmer it for about 15-20 minutes to let the flavors develop then season to taste, and add fresh herbs if you want (thyme would work best). You may need to add a slurry to thicken the sauce up.