Preheat oven to 375 degrees F.
Pour 2 inches of water into a large saucepan set over medium-high heat. Place a steamerbasket inside and add the cauliflower florets. Cover and steam for 5-6 minutes, or until thecauliflower is tender but still slightly firm.
Meanwhile, large cast iron skillet to medium high heat. Add sausage and break apart using a wood spoon. Once partially cooked, add the onion, and mushroom and saute until browned and everything is cooked through, 5-7 minutes.
Add garlic and cook 1 additional minute, until fragrant.
Add pasta sauce to skillet. Season with 2 teaspoons Italian seasoning, salt, and pepper, stirring well (if using unseasoned sausage).
Add the cauliflower to the skillet and stir to combine. Remove from heat and sprinkle the top evenly with mozzarella cheese. Evenly place pepperoni (or desired toppings) on top of the cheese.
Bake, uncovered, for 10-12 minutes or until cheese is melted and the edges are golden brown. Sprinkle with remaining Italian seasoning and serve.