Melt butter in a skillet over medium heat. Add garlic, salt, black pepper, and thyme; cook until fragrant, about 30 seconds.
Add mushrooms; sauté until tender, around 5 minutes.
Pour in wine, beef broth, and bay leaf; simmer until sauce reduces by a quarter, roughly 20 minutes.
Whisk water and cornstarch together in a bowl. Stir into sauce. Adjust heat to medium-low; let it thicken for 2 to 3 minutes.
Remove and discard bay leaf before serving.
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