Preheat the oven to 375°F.

Spray a 9x13-inch baking dish with nonstick cooking spray. Set aside.

Finely the chop onion. Slice the mushrooms. Mince the garlic.

Set a large pot over medium-high heat and add the butter. Once melted, add the onion and mushrooms. Saute until vegetables are browned and softened, 6-7 minutes. Add the minced garlic and continue cooking until fragrant, about 30 seconds. Add the riced cauliflower to the pot and cook, stirring constantly, for 1 minute longer. Remove pot from heat and transfer vegetable mixture to a large mixing bowl.

Add the chicken, sour cream, dried parsley, garlic powder, paprika, salt, black pepper, and shredded cheese to the bowl. Fold into the vegetable mixture until all the ingredients are thoroughly combined.

Transfer the chicken and vegetable mixture to the prepared baking dish. Sprinkle the shredded cheese evenly over the top.

Bake the casserole on middle rack of oven until cheese is golden brown and dish is gently bubbling at the edges, 30-35 minutes.

Check to see that casserole is done. Remove from oven or add time as needed.

Allow the casserole to rest for 10-15 minutes before serving.