Prep the veggies: First, prep them by brushing the mushrooms with a wet paper towel and slicing them, chopping the onion, and mincing the garlic. Set aside.
Sauté the Vegetables: Add a little water or vegetable broth in a large non-stick pan for water-sautéing. Add the onions, a pinch of salt, and sauté them for around 5 minutes or until the onion is translucent and lightly browned. Next, add minced garlic and sauté for 2 minutes until fragrant. And last, add the sliced mushrooms. Cook over medium heat until the mushrooms release their juices. Their water evaporates, and they start to brown nicely around 10 minutes. Deglaze with a little bit of water or vegetable broth.
Add Flavorings: Stir in the soy sauce, nutritional yeast (if using), thyme, and rosemary*. Cook for another 2-3 minutes to let the flavors meld together.
Create the Gravy Base: Pour the vegetable broth and bring the mixture to a low simmer.
Thicken the Gravy: In a small bowl, prepare the slurry, whisking together the cornstarch and water until smooth. Slowly pour this mixture into the simmering gravy, stirring constantly. Cook and stir until the gravy reaches your desired thickness (5-10 minutes).
Adjust Seasonings: Taste and adjust the seasonings, adding salt, pepper, or herbs as needed.
Serve: Serve hot over mashed potatoes or your favorite vegan dishes. Garnish with fresh, finely diced parsley (optional).