Preheat oven to 375°F and lightly grease a baking dish.
In a large skillet, cook the sausage with Italian seasoning over medium heat until browned. Drain any excess grease. Stir in the diced tomatoes and cook for 3–4 minutes.
Transfer the mixture to the prepared baking dish. Top evenly with mozzarella cheese, pepperoni, mushrooms, black olives, and green pepper.
Bake for 20–25 minutes, or until the cheese is melted and bubbly.
Remove from oven and let cool slightly before serving.