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.