In a medium microwave-safe bowl, microwave the cauliflower rice, covered, on High for 4 minutes.
Meanwhile, coat a Dutch oven with cooking spray and heat over medium-high. Cook the shallots and garlic, stirring, for 3 minutes. Add the mushrooms and thyme. Cook, stirring occasionally, until the mushrooms are tender and the liquid has evaporated, 6 to 7 minutes. Stir in ½ tsp salt and ¼ tsp black pepper.
In a blender, blend the stock, 1 cup of the mushroom mixture, the cauliflower rice, and remaining ½ tsp salt and ¼ tsp black pepper on high speed until smooth, about 1 minute. Pour the soup into a large pot and stir to combine. Heat over medium until thoroughly heated through, about 2 minutes.
Ladle the soup into bowls. Garnish with the remaining mushroom mixture, thyme, and black pepper.
Serving size: 1 cup