Wipe mushrooms clean with a damp cloth or quickly rinse and pat dry. Trim stems and slice mushrooms into 1/4-inch to 1/2-inch slices for even cooking.

Sweat onion

Heat a large skillet over medium heat. Add 1 tablespoon butter and the olive oil. When the butter foams, add the diced onion and a pinch of salt. Cook, stirring occasionally, until translucent and soft, about 5–7 minutes.

Brown mushrooms

Increase heat to medium-high and add remaining 1 tablespoon butter. Add the sliced mushrooms in a single layer (work in batches if needed to avoid crowding). Let mushrooms sear without stirring for 2–3 minutes so they brown, then stir and continue cooking until they release liquid and it evaporates and mushrooms are golden brown, about 6–8 minutes total.

Add garlic

Push mushrooms to the side and add the minced garlic to the hot pan. Cook 30–45 seconds until fragrant, then stir into the mushrooms.

Liquids & Cream

Deglaze the pan with the white wine (if using), scraping up any brown bits from the bottom. Let it reduce until mostly evaporated, about 1–2 minutes. If not using wine, add 1 tablespoon broth and scrape.

Add remaining broth and bring to a gentle simmer. Reduce heat to medium-low and stir in the heavy cream. Let the mixture simmer gently until the sauce thickens slightly, about 3–5 minutes. If the sauce becomes too thick, stir in up to 2 tablespoons whole milk to reach desired consistency.

Seasoning & Finish

Season the creamy mushrooms with the freshly ground black pepper, sea salt (taste and adjust), and thyme. Stir in the Dijon mustard if using for extra depth.

If using, stir in the grated Parmesan until melted and incorporated. Simmer another 1 minute to meld flavors.

Remove pan from heat and taste for seasoning, adjusting salt and pepper as needed. Sprinkle chopped parsley over the top for freshness.

Serve immediately as a side, over toasted bread, pasta, mashed potatoes, or steak.