Instructions:
- Melt half the butter in a large, heavy-bottomed skillet over medium heat. Add minced shallots and cook gently until softened and translucent (about 3-4 minutes).
- Increase heat to medium-high. Add the remaining butter and the sliced mushrooms. Sauté without stirring initially to allow them to release moisture and brown deeply (about 5-7 minutes). Mushrooms should shrink significantly and achieve a deep caramel colour.
- Stir in the minced garlic and thyme. Cook for 60 seconds until fragrant.
- Sprinkle the flour over the mushroom mixture. Stir continuously for 1 minute to cook out the raw flour taste, creating a roux base.
- Pour in the white wine (if using). Scrape up any browned bits stuck to the bottom of the pan (fond). Let the wine bubble and reduce by half.
- Slowly whisk in the cold stock until fully incorporated, ensuring no lumps form. Bring the mixture to a gentle simmer.
- Reduce heat to low. Stir in the heavy cream and Dijon mustard. Let the sauce simmer very gently for 5-8 minutes, stirring occasionally, until the sauce coats the back of a spoon (Nappé consistency).
- Remove from heat. Taste and aggressively season with salt and plenty of freshly ground black pepper. Stir in half of the fresh parsley.
- Serve immediately over your chosen main dish, garnished with the remaining parsley.