Begin by cleaning and drying the mushrooms thoroughly. Remove the stems and slice the caps into thick pieces. Set aside.

In a large skillet, heat 1 tablespoon of butter over medium heat. Add the chopped shallots and sauté until softened, approximately 3 minutes. Add a third of the sliced mushrooms to the pan, cooking for 2–3 minutes until they are tender. Remove from the pan and transfer to a bowl. Repeat this process in two additional batches, adding 1 tablespoon of butter each time, until all mushrooms are cooked. Set aside.

Increase the heat to medium and add garlic, rosemary, and wine to the same skillet. Stir to combine, scraping up any browned bits from the bottom of the pan. Let the mixture simmer until it has reduced by about 75%, stirring occasionally. Once the mushrooms and shallots have released some of their juices, add them back into the pan.

Add the chicken broth and heavy cream to the pan. Season with salt and pepper to taste, and allow the mixture to simmer for 4–5 minutes, until the sauce has reduced by half. Stir occasionally, ensuring the cream does not scorch.

If the sauce is too thin, create a slurry by mixing the cornstarch with cold water. Gradually whisk the slurry into the sauce, a little at a time, until it reaches the desired thickness. To thin the sauce, add chicken broth one tablespoon at a time.

Garnish with chopped parsley if desired and serve the sauce immediately over your steak.