Preheat oven to 425°F (220°C).
In a large skillet over medium heat, add olive oil and cook Italian sausage, breaking it apart with a spoon, until browned and cooked through, about 7-8 minutes. Remove sausage from skillet and set aside.
In the same skillet, add diced green bell pepper, diced onion, and sliced mushrooms. Sauté until vegetables are tender, about 5-6 minutes. Add minced garlic and cook for an additional 1 minute. Remove from heat.
In a bowl, combine crushed canned tomatoes, dried oregano, dried basil, salt, and black pepper. Stir well to make the tomato sauce.
Spread half of the shredded mozzarella cheese evenly in the bottom of a deep dish pizza pan or a 9-inch round cake pan.
Layer the cooked Italian sausage evenly over the cheese.
Add the sautéed vegetables on top of the sausage layer.
Pour the tomato sauce evenly over the vegetables and sausage.
Top the pizza with the remaining mozzarella cheese.
Bake in the preheated oven for 35-40 minutes, or until the crust is golden and the cheese is bubbly and slightly browned.
Remove from oven and let cool for 5 minutes before slicing and serving.