Clean the mushrooms and slice them evenly. Dice the onion and mince the garlic.
Heat the olive oil in a large skillet over medium-high heat. Add the mushrooms and cook for 8 to 10 minutes, stirring occasionally, until their moisture evaporates and they develop golden-brown patches.
Add the onion and cook for 4 to 5 minutes over medium heat, until softened and lightly golden.
Stir in the garlic and dried thyme. Cook for 30 to 60 seconds until fragrant.
Sprinkle in the flour and stir continuously for 1 to 2 minutes, coating the vegetables and cooking away the raw flour taste.
Slowly whisk in about ½ cup of vegetable broth until smooth. Gradually add the remaining broth.
Stir in the soy sauce and black pepper. Bring to a gentle simmer.
Simmer for 8 to 12 minutes, stirring regularly, until the gravy thickens and coats the back of a spoon.
Taste and add salt as needed. If the gravy is too thick, add a little warm vegetable broth. If it is too thin, continue simmering or add a small amount of cornstarch slurry.
Serve warm over mashed potatoes, stuffing, roasted vegetables, or a plant-based main.