Heat 2 tablespoons butter in a skillet over medium-high heat. Add mushrooms and cook 2-3 minutes without stirring or until they begin to caramelize. Stir and cook until cooked through. Remove from pan and set aside.
Add the last tablespoon of butter to the skillet. Add onion, garlic, and thyme. Cook over medium heat until softened, about 4-5 minutes.
Add riced cauliflower, seasoning salt and pepper to taste, and stir to combine.
Cook 9-11 minutes or until tender but not mushy.
Add cream, parmesan cheese, parsley, and mushrooms (along with any mushroom juice that may have accumulated on the plate).
Cook 2-3 minutes or until heated through and cheese is melted.
Drizzle with truffle oil if desired and serve warm.
Notes
Leftovers will keep in an airtight container in the refrigerator for 5 days, and in the freezer for 1 month.