Heat a large skillet over medium-high heat. Add the olive oil, then the mushrooms. Spread them in an even layer and let cook, undisturbed, for 4 minutes. Add the onion, garlic, salt, pepper, and thyme, and stir to combine. Cook 2 minutes more.
Reduce the heat to medium. Add the butter to the skillet. Once melted, sprinkle the flour evenly over top of the mushrooms. Stir until the flour mixture is lightly toasted, about 1 minute.
Add the vegetable stock and soy sauce. Scrape the bottom of the skillet to remove all the brown bits and stir until smooth. Increase the heat to medium-high to return to a simmer, then reduce heat to medium and let the gravy simmer for 5 minutes.
Remove from the heat and stir in the parsley.
