Cook Cauliflower Rice: In a large pan over medium-high heat, cook frozen cauliflower rice until softened. Transfer cauliflower to bowl and set aside.
Cook Mushrooms, Onions, And Seasonings: Reduce stovetop heat to medium and in same pan, melt butter. Add mushrooms and onions and saute until mushrooms are browned, onions are translucent and slightly browned, and all liquid has evaporated, about 10 minutes, stirring occasionally. Add thyme sprigs, salt, garlic powder, and pepper and sauté until fragrant, about 1 minute.
Add Cauliflower Rice Back Then Simmer: Carefully remove thyme sprigs from pan. Increase stovetop heat to medium-high, return cauliflower rice to pan, then pour in chicken broth and stir. Bring mixture to a simmer and allow to simmer until liquid has reduced slightly, about 3 minutes, stirring occasionally.
Add Cream And Cheese: Reduce stovetop heat to low before adding in heavy cream and grated Parmesan. Stir constantly until cheese melts, about 1 minute. Taste test and add additional seasoning, based on taste preferences.
Garnish And Serve: Spoon risotto into bowls, garnish with chopped fresh thyme and pepper, and serve.