Combine the dried mushrooms and hot water in a large measuring cup or small bowl. Let them soak for at least 10 minutes.

Warm the olive oil in a medium pot over medium heat. Add the onion, cremini mushrooms, and garlic salt, and cook for 10 minutes, until the mushrooms are tender.

Lift the soaked mushrooms out of the liquid, chop them into small pieces, and add them to the pot. Save the soaking liquid, leaving any grit behind in the cup. Add the vegetable broth and bring to a boil.

Whisk the arrowroot into the reserved soaking liquid until smooth, then pour it into the pot. Lower the heat and simmer for 5 minutes, stirring often, until slightly thickened. Stir in the tamari and take the pot off the heat.

Preheat the oven to 400°F and line a baking sheet with parchment paper. Trim the leaves and the very bottom of the stem from the cauliflower. Cut the head in half through the stem, then cut each half into ¾- to 1-inch-thick slices.

Stir together the thyme, oregano, salt, and pepper. Arrange the steaks and any loose florets on the baking sheet a few inches apart, mist with oil, and sprinkle with the seasoning.

Bake for 15 minutes, flip, and bake for another 15 minutes, until golden brown and crisp at the edges.

While the cauliflower bakes, blend the gravy until smooth. For a chunkier gravy, set some of the mushrooms aside before blending and stir them back in. Rewarm the gravy over low heat.

Spoon the warm gravy over the cauliflower steaks and top with parsley.