Add dried mushrooms to a large pot or saucepan with the vegetable broth, bring to a boil, and allow to cook for about 1 minute. Turn off the heat and set aside.
Heat oil in a large skillet over medium heat. Once hot, add onion and sauté for 2-3 minutes. Then add garlic and herbs, and sauté for another minute, stirring frequently.
Meanwhile, in a small bowl, combine the plant-based milk and cornstarch and stir/whisk until lump-free.
Next, deglaze the skillet with tamari or soy sauce while stirring to scrape up any bits from the bottom of the pan.
Pour in the mushroom and broth mixture and stir to combine. Stir in the cornstarch milk mixture and let gravy simmer and thicken for at least 5 minutes. The longer it simmers, the thicker and more flavorful the mushroom gravy will become.
Using an immersion blender or blender, blend until smooth. If the gravy is too thick, add more vegetable broth. If it’s too thin, add more cornstarch and heat on the stovetop until thickened, stirring constantly.
Add salt and pepper, to taste.
Store leftovers covered in the fridge for up to 4-5 days. You can also freeze the gravy for up to 2 months.