Preheat oven to 350 degrees. Butter a 9X13 inch baking dish; set aside.
Cook Spaghetti according to package directions, just until al dente. Drain well and return cooked pasta to original pot it was cooked in. Add 1 jar of prepared marinara sauce and toss to evenly coat pasta.
In a large skillet, brown beef adding the Italian seasoning, salt and pepper, and cook until there is no longer any pink. Remove cooked beef to a paper towel lined plate; set aside. Drain and discard all but 2 tablespoon of fat from skillet. If you don't have 2 tablespoon 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, red peppers, and garlic to skillet and saute until they are soft. Remove from heat. Add cream cheese, sour cream, and Mozzarella to 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 baking dish and spread out evenly. Sprinkle top with Parmesan cheese.
Bake in preheat oven until casserole is baked through the center, about 25 minutes.
Remove casserole and top with the french fried onions and bake another 5 minutes, watching carefully so onions do not burn. SERVE!