Melt 1 tablespoon of the unsalted butter in a large skillet over medium heat.
Add 2 minced garlic cloves and cook until fragrant, about 45 seconds.
Add 8 to 10 ounces sliced mushrooms, 1 teaspoon finely chopped fresh thyme leaves, ¼ teaspoon finely chopped fresh rosemary leaves, ¼ teaspoon kosher salt, and ¼ teaspoon black pepper.
Cook until the mushrooms have released their moisture and are golden-brown, 6 to 8 minutes.
Add the remaining 2 tablespoons unsalted butter.
When melted, sprinkle 2 tablespoons all-purpose flour over the mushrooms and stir until all the mushrooms are covered in flour.
Cook until the flour is lightly toasted, about 2 minutes.
Add ¼ cup dry white wine and cook, stirring often, until the white wine is fully absorbed into the mushrooms.
Add 2 cups beef broth and cook, whisking frequently, until the flour is dissolved.
Simmer until the gravy thickens enough to coat the back of a spoon, 4 to 6 minutes.
Garnish with freshly cracked black pepper if desired.
