Thinly slice 8 ounces cremini or white button mushrooms. Mince 2 garlic cloves.
Melt 2 tablespoons unsalted butter in a medium skillet over medium heat. Add the mushrooms and sauté until browned, 8 to 10 minutes. Add the garlic and sauté until fragrant, about 30 seconds.
Add 1/2 cup beef broth, 1/2 cup water, 2 tablespoons heavy cream, 1 teaspoon Dijon mustard, 1/2 teaspoon Worcestershire sauce, 1/4 teaspoon dried thyme, and 1/4 teaspoon black pepper. Scrape up any browned bits from the bottom of the pan with a wooden spoon. Simmer until the sauce is reduced and thickened slightly, 4 to 6 minutes.
Spoon over cooked steak, and top with more black pepper, if desired.