Prepare the cauliflower. Bring a medium pot of water fitted with a steamer basket to a boil over medium-high heat. Add the cauliflower and steam, covered, for about 9 minutes, or until tender.

Place half of the steamed cauliflower in the bowl of a food processor. Add the coconut milk, nutritional yeast, oil and garlic, and process until the cauliflower is broken up. Add the remaining cauliflower and process until it reaches the consistency you prefer. Season with salt and pepper. Transfer the cauliflower to a medium serving bowl and set aside.

Prepare the gravy. Heat the coconut oil in a medium saucepan over medium-high heat. Add the onions and garlic and sauté, stirring frequently, for about 4 minutes, or until the onions are lightly brown. Stir in the mushrooms and sauté for 5 minutes more, or until the mushrooms are tender.

Next, add the vegetable broth, tamari and thyme to the mushroom-onion mixture. Bring the mixture to a boil over medium heat. Then lower the heat to medium-low and simmer, covered, stirring occasionally, until it thickens, about 25 minutes. Stir in the arrowroot starch and cook the sauce, stirring frequently, for another 5 minutes. Transfer the sauce to a blender and puree for a smooth consistency.

Season the sauce with salt and pepper. Pour the sauce over the reserved cauliflower and serve at once.