In a large measuring cup or a small bowl, combine 2 cups hot water with the dried mushrooms, then allow to sit for at least 10 minutes.
In a medium-size pot, warm the olive oil over medium heat. Add the onion, fresh mushrooms and garlic salt, and cook for 10 minutes, until the mushrooms are tender. Next, separate the dried mushrooms from the liquid (reserving the liquid for later), chop them into small pieces, and then add them to the pot with the cooked fresh mushrooms. Add the vegetable broth and bring to a boil. Sprinkle the arrowroot powder in the cup with the reserved liquid and whisk until smooth. Pour in the pot with the mushrooms and broth. Lower heat and allow to simmer for 5 minutes, until slightly thickened, while stirring intermittently. Remove from heat, season with tamari and set aside to cool.
Preheat oven to 400ºF and line a baking sheet with parchment paper. Using a large sharp knife, trim the leaves from the bottom of the cauliflower and cut off the very bottom of the stem. Cut the cauliflower in half down the middle, through the stem. Take one half of the cauliflower and cut it into ¾ to 1-inch thick slices—you should end up with 2-3 steaks, with some florets falling from the sides. Repeat with the other half of the cauliflower.
In a small bowl, combine the thyme, oregano, salt, and pepper. Arrange the cauliflower steaks on the baking sheet a few inches apart and lightly spray them with oil. Sprinkle the seasoning on top, and then bake in the oven for 15 minutes. Flip them over and continue to bake for another 15 minutes, until light golden brown and crispy.
While the cauliflower bakes, transfer the gravy to a blender and blend until smooth. If you prefer some larger pieces of mushrooms in your gravy, be sure to scoop them out before blending. Pour the gravy back into the pot and warm over low heat. When the cauliflower steaks are done, serve immediately with warm gravy. Top with fresh parsley, if desired, and enjoy!
Recipe makes approximately 4 cups of gravy, which will likely be more than you need. Feel free to save some for mashed potatoes or freeze it for later.