Preheat oven to 375ºF and grease a 9x13-inch baking dish.
Over medium-high heat, cook the ground beef in a large skillet for 5 minutes or until browned, drain if needed. Add the chopped onion and chopped bell peppers - cook for 5 more minutes until tender.
Add marinara sauce, Italian seasoning, and salt. Stir to combine. Simmer over medium heat for 10 minutes.
Meanwhile, cook the pasta according to the packaging instructions until al dente (the pasta should have a bite). Drain the pasta.
Add the pasta to the meat mixture and toss well.
Transfer mixture to the prepared baking dish. Sprinkle shredded mozzarella cheese on top.
Top with several pepperoni slices then cover the baking dish with foil.
Bake for 20 minutes. Remove foil and broil for an additional 5-10 minutes until the cheese is bubbly and golden brown.
Remove the casserole from the oven and let it cool for 5-10 minutes before serving.