Set the oven rack to the middle position—Preheat the oven to 375°F (190ºC).
In a large pot, bring water to a boil. Add the salt, stir to dissolve. Add in the spaghetti, cook according to package directions until al dente, about 10 minutes. Drain the noodles in a colander, and rinse with cool water for about 30 seconds to stop the cooking.
Heat olive oil in a large skillet or dutch oven over medium-low heat. Add the onion, garlic, and 1 teaspoon Italian seasonings, sauté until onions are translucent, about 3 minutes.
Add the marinara sauce, stir to combine. Cook until the sauce is hot, about 2 to 3 minutes. Turn off the heat and stir in the heavy cream. Season the sauce with more salt and pepper to taste.
Add the drained spaghetti to the sauce, stir to combine. Add the shredded chicken and mix. Transfer the pasta to a 13x9-inch baking dish.
Sprinkle the pasta with the shredded mozzarella and parmesan.
Cover with foil and bake until the cheese melts and the pasta is hot, about 15 to 20 minutes. For a more browned surface, remove the foil, and broil the casserole until lightly browned, about 2 to 5 minutes.
Garnish with chopped parsley, and serve immediately.