Preheat oven to 350ºF (177ºC). Butter a 9X13 inch baking dish; set aside.
Cook spaghetti (16 ounces) according to package directions, just until al dente. Drain well and return the cooked pasta to the original pot it was cooked in. Add 1 jar of prepared marinara sauce and toss to evenly coat the pasta.
In a large skillet, brown the beef (1 pound), add the Italian seasoning (1 tablespoon), salt (1 teaspoon), and pepper (1/2 teaspoon), and cook until there is no longer any pink. Remove the cooked beef to a paper towel-lined plate; set aside. Drain and discard all but 2 tablespoons of fat from the skillet. If you don't have 2 tablespoons of oil, add enough olive oil to compensate. (Just eyeball this. You only need enough oil to saute the veggies.)
In a medium bowl mix the cooked beef and remaining 1 jar of prepared marinara sauce.
Add onions (1 cup), red peppers (1/2 cup), and garlic (4 cloves) to skillet and saute until they are soft. Remove from heat. Add cream cheese (12 ounces), sour cream (1/4 cup), and Mozzarella (2 cups) to the skillet and stir until well combined. Set aside.
In prepared baking dish, add about 1/3 of a cup of sauce-meat mixture, and spread to cover bottom of dish.
Add all the spaghetti to baking dish and spread out evenly.
Add the cream cheese-veggie mixture to baking dish and spread out evenly.
Add the sauce-meat mixture to the baking dish and spread it out evenly. Sprinkle top with Parmesan cheese (1 1/2 cups).
Bake in the preheated oven until the casserole is baked through the center, about 25 minutes.
Remove the casserole and top with the french fried onions (1-2 cups) and bake another 5 minutes, watching carefully so the onions do not burn. SERVE!