optional toppings: chopped fresh basil or Italian parsley, shaved Parmesan cheese
Preheat the oven to 190 degrees.
Cook pasta in generously salted water until al dente according to package instructions.
While pasta is cooking, heat olive oil in a skillet over medium-high heat. Add the onion and saute for 5 minutes, until it is cooked and translucent. Add the garlic and oregano and cook for 1 minute more, stirring occasionally, until the garlic is fragrant. Remove from heat.
Once the pasta is finished cooking, drain the water and return the pasta back to the pot. Stir in the garlic/onion mixture, the marinara sauce, diced tomatoes, Mozzarella (reserve a small handful of the cheese for the topping), Parmesan and pepperonis. Mix until well-blended.
Transfer the pasta to a greased baking dish, and then sprinkle the remaining Mozzarella cheese on top. Bake uncovered for about 20-25 minutes, or until the cheese is melted and starts to brown around the edges.