If you're preparing the sauce for a steak, chicken, or pork, make sure to cook the meat first and then take it out and set aside to rest. While the meat is resting, prepare the sauce in the same pan.
The Sauce:
Prep the ingredients before starting to cook because this is a very fast moving recipe. You'll want all the ingredients be ready to add.
Slice mushrooms and shallots, and smash and mince the garlic. Measure all remaining ingredients and keep them close.
In the same pan where you cooked the meat, over medium heat, add a little more oil to the pan if needed.
Add sliced shallots and sauté until golden brown. Mix in minced garlic and sauté just until fragrant.
Add mushrooms and butter and stir. Let mushrooms cook for a few minutes before stirring. Stir just a couple of times as mushrooms cook until done.
Slowly pour in beef stock as you stir and scrape the bottom of the pan. Mix well.
Mix in Dijon mustard, salt, and pepper.
Bring it to simmer and simmer for a minutes or so. Lower the heat to medium-low and stir in heavy whipping cream and grated Parmesan cheese. Stir and let it gently simmer for a couple of minutes. (Make sure that the sauce does not simmer hard or especially comes to boil.)
Take off heat and serve over steaks (or other meats and pastas!)
Video
Notes
For the best sauce flavor, cook the meat first, then use the same pan to cook the sauce. All of those wonderful juices that you’ll get in the bottom of the pan after searing the meat will get incorporated into the sauce and that can’t be beat or replicated!
Sear the onions until browned. That will caramelize them and also make them more flavorful.
Use a combination of different mushrooms. This will give you more complex and rich mushroom flavor.
Try to use freshly grated parmesan cheese. It has the richest flavor and you can usually buy fresh grated parmesan cheese in your deli section of the grocery store, especially if you happen to have a nice cheese department.
Remember, DO NOT let heavy cream boil, only gentle simmer. So don’t raise the stove-top heat more than medium, but it is better to cook over medium-low.