Preheat oven 400F.
In a large oven safe skillet, heat oil on medium-low heat. Once the oil has heated, add the onion and cook for 2-3 minutes, until the onions are translucent.
Add the garlic, bell pepper, and mushrooms. Cook for 3-4 minutes, until the vegetables have softened a bit.
Add the turkey sausage, breaking up into smaller pieces as it browns. Cook until it is no longer pink, about 5 minutes.
Add the zucchini and cook until the zucchini has softened slightly, about 3 minutes.
Add the chopped pepperoni, pizza sauce, salt, oregano, and pepper. Mix to combine all of the ingredients. Stir in ½ cup of cheese. Once fully combined, top the mixture with the other ½ cup of cheese and a few slices of pepperoni.
Bake in the preheated oven for 10 minutes, until the cheese is melted and the casserole is slightly bubbling.