Ready to bring this comforting dish to life? Follow these steps:

1. Preheat your oven to 400°F (200°C).
2. In a large skillet, heat olive oil over medium heat. Add the chopped onion and garlic, sautéing until fragrant and translucent, about 3 minutes.
3. Add the Italian sausage to the skillet, breaking it up with a spoon. Cook until browned and cooked through, about 5-7 minutes.
4. Stir in the red bell pepper, mushrooms, oregano, and crushed red pepper flakes. Cook until the vegetables are softened, about 4 minutes.
5. Pour in the marinara sauce, stirring to combine. Let it simmer for another 3 minutes to meld the flavors.
6. Transfer the sausage and vegetable mixture to a deep-dish pie plate or a small casserole dish.
7. Sprinkle the mozzarella and Parmesan cheese evenly over the top.
8. Roll out the pizza dough and place it over the filling, tucking the edges into the sides of the dish to seal.
9. Using a knife, cut a few slits in the top of the dough to allow steam to escape.
10. Bake in the preheated oven for 20-25 minutes, or until the crust is golden brown and the filling is bubbly.
11. Let it cool slightly before garnishing with fresh basil leaves.