1. To start, in a sizable measuring cup or a compact bowl, combine the dried mushrooms with the two cups of hot water and let it sit undisturbed for a minimum of 10 minutes; this will allow the mushrooms to rehydrate.
2. Next, in a pot of medium size, heat the olive oil over medium heat. Add the onions, the fresh mushrooms, and the garlic salt and cook them for 10 minutes, or until the fresh mushrooms have become tender. After separating the dried mushrooms from their liquid (saving the liquid for later), chop them finely. Add them and the cooked fresh mushrooms to the pot. Include the vegetable broth and bring it to a boil. Then, in a cup, combine the arrowroot powder and the reserved liquid, whisking until smooth. Pour it into the pot with the mushrooms and broth. Reduce heat and simmer for 5 minutes, stirring occasionally, until the mixture has thickened slightly. Remove it from the heat, season with tamari, and set it aside to cool.
3. Set your oven to 200°C. Line a baking sheet with parchment paper. Using a sharp knife, remove the leaves from the cauliflower's base and cut off the stem's bottom. Cut the cauliflower in half lengthwise, through the stem. Take one half and cut it into slices that are 3/4 to 1 inch thick—you should have 2 or 3 steaks, with florets falling off the sides. Do the same with the other half.
4. In a small bowl, combine the thyme, oregano, salt, and pepper. Arrange your prepared cauliflower steaks on the lined sheet, spacing them a few inches apart. Lightly spray them with oil and sprinkle the seasoning on top. Bake in the oven for 15 minutes. Flip them over and continue baking for 15 more minutes, until they are light golden brown and crispy.
5. Meanwhile, you can transfer the gravy to a blender and blend it until smooth. If you prefer larger mushroom pieces, remove them before blending. Pour the gravy back into the pot and heat it over low heat. When the cauliflower steaks are ready, serve them immediately with warm gravy. If desired, garnish with fresh parsley and enjoy!