Heat cauliflower rice in a medium bowl in the microwave for 8 minutes. Carefully transfer mixture to a clean kitchen towel or cheesecloth. Squeeze cauliflower over sink to drain liquid from cloth. Transfer cauliflower to a medium bowl to cool slightly.
Add egg, Cheddar cheese, Italian seasoning, salt, and pepper to cauliflower and mix to combine well.
Transfer mixture to an 8- x 8-inch baking dish lightly coated with cooking spray. Spread mixture over bottom and up sides of pan to about a 1/4-inch thickness.
Bake at 450° for 22-25 minutes or until golden brown.
Spread pizza sauce evenly over bottom of crust. Top with Mozzarella cheese and pepperoni slices.
Bake the keto pizza casserole for an additional 10 minutes to melt cheese.
Notes
You can use regular pepperoni or turkey pepperoni depending on your preferences.
If you like a sweeter sauce, feel free to add keto-friendly sweetener to taste to your sauce before adding it to the dish.