Wash hands with soap and water.
Preheat the oven to 375 °F.
Gently scrub cauliflower with a clean vegetable brush under cold running water. Dry with a clean towel.
Grate cauliflower with a box grater or food processor.
In a large nonstick skillet over medium heat, cook the cauliflower for 10 to 12 minutes until tender and the excess moisture removed. Allow the cauliflower to come to room temperature.
In a large mixing bowl, combine 2 cups of cauliflower along with egg, Parmesan cheese, ½ cup mozzarella cheese, oatmeal, ¼ teaspoon garlic powder, ¼ teaspoon onion powder, black pepper, and salt. Mix to combine.
Wash hands with soap and water after handling raw eggs.
Line pizza pan or baking sheet with parchment paper.
On the parchment paper, spread the cauliflower dough to form the crust, about 9-inches round and about ¼-inch thick.
Bake the crust for 15 to 20 minutes until golden brown. Remove from the oven and let cool.
While the crust bakes, mix to combine the tomato sauce, tomato paste, ¼ teaspoon garlic powder, ¼ teaspoon onion powder, and parsley in a small bowl.
Gently rub the mushrooms, spinach, bell pepper, and tomato under cold running water. Dry with a clean towel and slice.
Top the pizza crust with tomato sauce mixture, ½ cup mozzarella cheese, mushrooms, spinach, bell pepper, and tomato slices.
Bake in the oven an additional 5 to 10 minutes until the cheese is melted.
Allow the pizza to cool for 2 minutes and cut into 8 slices.