Saute butter, mushrooms, and onions over medium heat for 5 to 7 minutes or until the mushrooms are tender and the onions soften.
Add the garlic, and continue to saute for an additional 30 seconds.
Pour in the white wine to deglaze the pan. Reduce the heat to low-medium, and allow the sauce to simmer for 3 to 4 minutes.
In a separate bowl, whisk together vegetable broth, Worcestershire sauce, flour, Dijon mustard, and thyme.
Pour the mixture into the pan, and stir to combine.
Add the sour cream, and stir until all the ingredients are well incorporated. Taste, and season with salt and pepper.
Serve over egg noodles or mashed potatoes. Garnish with fresh parsley.
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