Heat up a saucepan over medium heat. When hot, add the butter and shallots and sauté until the shallots are translucent - about 2 to 3 minutes.
Remove the pan from the heat and deglaze it with the wine.
Return the pan back to the heat, add the cracked peppercorns and rosemary. Cook until the wine is reduced by half.
Add the demi glace or chicken based demi glace to the pan and stir or whisk to combine ingredients.
Continue reducing the sauce down until it is thick enough to coat the back of a spoon.
Optional - some chefs like to strain their sauces through a fine meshed strainer or cheesecloth at this point to make their sauces more refined. I don't but it is entirely up to you.
Taste the sauce and adjust seasonings with salt and pepper.
Notes
Serve with beef, veal, lamb, chicken or duck depending on what demi you went with.