Chop the onion and red pepper. Mince the garlic. Preheat your oven to 375°.
Cook spaghetti noodles in salted boiling water until al dente. Drain.
While pasta is cooking, melt the butter in a large oven-safe skillet over medium heat. Add the onion and red pepper; cook and stir until tender, about 5 minutes.
Add garlic, flour, and Italian seasoning. Cook and stir for another 1-2 minutes.
Stir in the chicken broth, cream salt, and pepper; cook and stir until thick and bubbly.
Add the Rotel, salt, pepper, parmesan cheese, shredded chicken, cooked spaghetti, and about 1 cup of the shredded cheese.
Stir until well combined. Bake at 375 degrees for 15 minutes, or until bubbly. Sprinkle the last cup of cheese on top of the casserole and bake for 5 more minutes, or until the cheese melts.
Notes
-If you don't have an oven-safe skillet, you can transfer the casserole to a greased 9x13" baking dish. It will take a few more minutes to cook.