Set the oven rack to the middle position and preheat the oven to 375°F.

In a large pot, bring 4 quarts of water to a boil. Add 1 tablespoon of kosher salt and stir to dissolve.

Add in 1 pound of spaghetti and 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 2 tablespoons of olive oil in a large skillet or Dutch oven over medium-low heat.

Add ½ cup of finely chopped yellow onion, 1 tablespoon of minced garlic, and 1 teaspoon of Italian seasoning. Sauté until the onions are translucent, about 3 minutes.

Add in 2 cups of marinara sauce and stir to combine. Cook until the sauce is hot, about 2 to 3 minutes.

Turn off the heat and stir in ½ cup of heavy cream. Season the sauce with more salt and pepper to taste.

Add the drained spaghetti to the sauce and stir to combine. Mix in 2 cups of cooked shredded chicken.

Transfer the pasta to a 9 x 13-inch baking dish.

Sprinkle the pasta with 1 ½ cups of shredded mozzarella cheese and ½ cup of shredded parmesan cheese.

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 1 tablespoon of chopped parsley and serve immediately.