Add chopped Portabella mushrooms and cook for 5-7 minutes until mushrooms release moisture and become tender.
Stir in flour and cook for another 1-2 minutes to make a roux.
Slowly pour in vegetable broth, stirring constantly until the mixture thickens.
Add soy sauce and thyme, and simmer for 5 more minutes, adjusting seasoning with salt and pepper.
Serve hot over mashed potatoes.
Notes
For a vegan version, simply ensure that your vegetable broth is plant-based and use olive oil instead of butter. Adjust thickness by adding more or less broth, depending on your preference.