• 12–16 oz mixed mushrooms (cremini + shiitake and/or oyster is ideal; quarter large ones, slice medium, tear oysters)
• 2 Tbsp unsalted butter + 1 Tbsp neutral oil (or all butter)
• 1 medium shallot, minced
• 2 garlic cloves, minced
• 2–3 sprigs fresh thyme (or ¾ tsp leaves)
• ¾ cup dry red wine (Cabernet, Merlot, Côtes du Rhône, or whatever you’d drink with the steak—avoid sweet or oaky cheap cooking wine)
• ½ cup low-sodium beef stock
• 1 tsp Dijon mustard (optional but recommended)
• ½ tsp balsamic vinegar or a squeeze of lemon at the end (optional, for lift)
• 2 Tbsp cold unsalted butter, cubed (for finishing)
• Kosher salt and black pepper
• Chopped parsley
Method
1. Brown the mushrooms properly.
Heat a wide skillet (12-inch stainless or cast iron) over medium-high until hot. Add oil + 2 Tbsp butter. When foaming, add mushrooms in a single layer. Don’t crowd—work in two batches if needed. Leave them alone 2–3 minutes so they sear, then toss. Cook until they release liquid, the liquid evaporates, and the edges go deep golden, 8–10 minutes total. Season with salt late so they don’t steam.
2. Aromatics.
Push mushrooms to the side. Add shallot to the empty spot; cook 1 minute until translucent. Add garlic and thyme; 30 seconds until fragrant. Stir everything together.
3. Deglaze and reduce.
Pour in the wine. Scrape the fond. Boil hard until the wine is reduced by about half and looks syrupy on the mushrooms, 3–4 minutes. Add beef stock. Reduce again until the sauce coats a spoon and the mushrooms look glazed, not swimming, 4–6 minutes. Stir in Dijon if using.
4. Finish off heat.
Kill the heat. Whisk in the cold butter cubes one at a time until the sauce turns glossy (monter au beurre). Taste. Add pepper, a pinch more salt, and balsamic or lemon if it tastes flat. Pull thyme sprigs. Fold in parsley.
Spoon over rested steak so the juices mix into the sauce.